Displacement sensor

Abstract

The invention provides a method for establishing an amount of displacement of a first and a second object with respect to one another, comprising: - generating a phase-synchronised radio-frequency signal which is signal phase-synchronised using a clock signal having an accuracy of at least 10-10-th of a second, in particular at least 10-11-th of a second, more in particular at least 10-12-th of a second; - transmitting said phase-synchronised radio-frequency signal into a resonance cavity having the first and second object coupled to opposite ends of said cavity; - extracting an electromagnetic cavity signal from an end of said cavity; - extracting an radio-frequency interference signal from said electromagnetic cavity signal, and - determining a cavity length from the radio-frequency interference signal, and determining from said cavity length said amount of displacement.

Description

[0001] DISPLACEMENT SENSOR

[0002] Field of the invention

[0003] The invention relates to a distance sensor and a method for measuring a displacement.

[0004] Background of the invention

[0005] Known displacement sensors for accurately measuring displacement can roughly can be divided into several types. A first type is based on laser beams and interferometers. A second type is based on change of capacitance. Furthermore, Piezoelectric sensors are available. Piezoelectric sensors are in general highly nonlinear and do not provide a high precision, often limited to 10 nm. Piezoelectric sensors are also highly sensitive to temperature and humidity variations. It is also prone to hysteresis.

[0006] Further distance sensing can be done using optical encoders and / or linear encoders. Interpolation is needed for a resolution up to nanometres.

[0007] With respect to capacitance positioning, reference is made to “The Nano Positioning Book” by Thomas R Hicks and Paul D Atherton, and "Position Sensors for Nano positioning" by Andrew J. Fleming and Kam K. Leang, published in Nano positioning Technologies, pp 245-294.

[0008] In particular, capacitive displacement sensor are discussed in for instance W02002008697, which in its abstract states: ’’The invention relates to a displacement measuring system (1) that is provided with a capacitive sensor (7a, 7b) configured as a differential capacitor (18). The partial capacitors (Cl, C2) thereof have capacitances that depend on the position to be determined. A processing device (5) that is provided with, for example, a sigma / delta demodulator determines the displacement to be measured. The partial capacitors (Cl, C2) are periodically controlled by binary signals, the control signals of the one partial capacitor (Cl) being transmitted in phase-shifted form with respect to the other partial capacitor (C2). The processing device (5) determines which control signals are used for evaluation. A switch unit (22) allows transmission of pertaining reception signals to the processing device (5) within time frames that are synchronized with the flanges of the control signals and suppresses all other signals. The displacement measuring system (1) reduces interpolation errors at high dissolutions and long interpolation periods. “

[0009] Capacitance type measuring device for absolute measurement of positions is also discussed in US5440501A, which in its abstract states: “A capacitance-type measuring device for absolute measurement of positions is disclosed, which comprises a displacement sensor having a fixed element and a movable element, the movable element being capacitance-coupled to the fixed element and relatively movable against the fixed element, the displacement sensor being adapted to output signals corresponding to relative positions of the movable element against the fixed element, a signal processing circuit for processing the output signals of the displacement sensor and outputting an absolute measurement value corresponding to displacement of the movable element against the fixed element, a control circuit for controlling the operations of the displacement sensor and the signal processing circuit, and a power supply for supplying electric power to the displacement sensor, the signal processing circuit, and the control circuit, wherein the control circuit activates the signal processing circuit at a suitable interval so as to perform an intermittent measurement operation.”.

[0010] In these capacitance-based sensors, capacitance using a reference capacitance is determined. This limits the performance of the sensor to the performance of the reference capacitor.

[0011] Laser interferometers are based upon Michelson interferometers and often used in nano positioning due to the availability of highly stable and coherent laser sources. A laser is extremely sensitive to the air flow due to pressure difference. The sensitivity is higher due to variation of refractive index due to different environmental changes like airflow, which acts as a false signal of motion. This can add on as a noise source to these interferometers. Due to this reason, they are mostly operated in hermetically sealed or vacuum conditions.

[0012] Summary of the invention

[0013] At least one disadvantage of prior art is illustrated above. Hence, it is an aspect of the invention to provide an alternative displacement sensor, which preferably further at least partly obviates one or more of abovedescribed drawbacks.

[0014] There is provide a method for establishing an amount of displacement of a first and a second object with respect to one another, comprising:

[0015] - generating a phase-synchronised radio-frequency signal which is signal phase- synchronised using a clock signal having an accuracy of at least 10-10-th of a second, in particular at least 10-11-th of a second, more in particular at least 10- 12-th of a second;

[0016] - transmitting said phase-synchronised radio-frequency signal into a resonance cavity having the first and second object coupled to opposite ends of said cavity;

[0017] - extracting an electromagnetic cavity signal from an end of said cavity;

[0018] - extracting an radio-frequency interference signal from said electromagnetic cavity signal, and

[0019] - determining a cavity length from the radio-frequency interference signal, and determining from said cavity length said amount of displacement.

[0020] There is further provided displacement sensing device for establishing an amount of displacement from a first and a second object with respect to one another, comprising:

[0021] - a clock signal input for providing a clock signal to the displacement sensing device, said clock signal having an accuracy of at least IO'10of a second, in particular at least 10'11of a second;

[0022] - a radio-frequency signal generator for providing a radio-frequency source signal, the radio-frequency generator phase coupled to the clock signal input in a synchronized manner for generating a phase-stabilized radio-frequency signal;

[0023] - an electromagnetic resonance cavity electromagnetically coupled to the radiofrequency generator, said electromagnetic resonance cavity having one cavity end coupled to the first object and a second cavity end coupled to the second object;

[0024] - a radio-frequency interferometer functionally coupled to the clock-stabilized radiofrequency signal and to the reflected radio-frequency signal for providing an output signal that is proportional to the displacement.

[0025] In the method and displacement sensing device that is proposed here, a radiofrequency or RF source is used that is phase synchronized to the atomic clock, a most stable reference of time. This stabilized RF source is subsequently used for measuring the position to a few nanometres accuracy by utilizing the stabilized RF signal as a ruler, measuring the change of phase of the RF signal due to the distance change. The currently proposed assembly or sensor reduces all the above noise sources and allows in specific embodiments a performance at least 100 better than these known technologies.

[0026] The current method and assembly can be build compact and simple. Ut uses few components, and each component is relative simple and common. It allows building a sensor that is cheaper and its performance can be 10-100 times better.

[0027] A current sensor can be self-calibrated due to the use of a calibrated ultra-stable RF source as a reference.

[0028] In theory, this method can be used to make a sensor that can have a measurement range of larger than 100mm max and a resolution better than lOnm, in particular even better than 1 nm. Capacitance sensors typically give a maximum measurement range of around 1mm. The large performance range of this sensor opens new applications like 3D printing systems, machining and welding instruments etc., but also application in wafer processing, wafer steppers, wafer positioning and the like.

[0029] In the current method and assembly using an RF source, the sensitivity to environmental changes like airflow was found 1000-10000 times lower than known sensors due to the very low change of refractive index (relative permittivity) of air at RF frequencies.

[0030] The absolute precision of a laser interferometric sensor is derived from the frequency stability. The potential of reaching sub-nanometre precision and accuracy is heavily limited by normal lasers unless we you a high stability low noise system. Whereas using a stable RF can easily beat this limit offered by laser interferometers with in terms of possible precision and accuracy with much cheaper and compact systems.

[0031] With respect to radio frequency or RF, see for instance https.7 / en. wikipedia.org / wiki / Radio_frequenc , that defines radio frequency or RF to a frequency of between 20 kHz and 300 GHz.

[0032] In some (older) textbooks, like for example David J. Griffiths, INTRODUCTION TO ELECTRODYNAMICS, Fourth Edition, page 396, “RF” is defined as having a frequency of between 103and 105Hz (1 kHz - 100 kHz, a wavelength of between 104-l 06metres).

[0033] In the current context, preference is made to define RF as electromagnetic radiation having a frequency of between 20 kHz and 300 GHz. This definition includes RF frequency in UHF, L, S, X, Ku, K, and Ka bands.

[0034] With respect to an RF reflector, currently a copper layer of less than 1 mm thickness integrated into a PCB is used. However, the layer thickness as well as the used material are selected based upon on the frequency bands of operation. Thus, each domain needs an optimized solution. The higher the (radio) frequency, the better the sensor, and the easier to reflect.

[0035] With respect to an atomic clock, it usually has an uncertainty better than 10’11. However a better (atomic) clock can be used, for instance having an uncertainty of better that 10'12-10'13. In current testing, a Rubidium frequency standard was used, see for instance https: / / www.thinksrs.com / products / fs725.html or http s : / / www. thinksr s . com / products / prs 10. html .

[0036] With respect to a phase-locked loop (also referred to as PLL), the following. A PLL used for the current device or method comprises in an embodiment a phase error detection unit followed by a loop filter and a phase actuator to provide feedback on the frequency / phase of the final output frequency. The actuator can be a voltage-controlled oscillator, a direct digital synthesizer, or any other digital frequency synthesizing method. The clock frequency (10 MHz) is multiplied by an integer value, for example, 11 times, to obtain 330 MHz while still performing a phase-locked loop on the 10 MHz.

[0037] Within a modern RF signal generator there are a number of major circuit blocks or devices:

[0038] Oscillator: This can be any form of oscillator, but today it would almost certainly be formed from a frequency synthesizer. This oscillator would take commands from the controller and be set to the required frequency.

[0039] Amplifier: The output from the oscillator will need amplifying. This will be achieved using a special amplifier module. This will amplify the signal, typically to a fixed level. It would have a feedback loop to maintain the output level accurately at all frequencies and temperatures. This feedback loop is closely controlled because the accuracy of the final output is depending on it. Attenuator: An attenuator is placed on the output of the signal generator. This serves to ensure an accurate source impedance is maintained as well as allowing the generator level to be adjusted very accurately. In particular the relative power levels, i.e. when changing from one level to another are very accurate and represent the accuracy of the attenuator. It is worth noting that the output impedance is less accurately defined for the highest signal levels where the attenuation is less, levels may often be adjusted in increments of 0. IdB over the range.

[0040] Control: Advanced processors are used to ensure that the RF and microwave signal generator is easy to control and is also able to take remote control commands. The processor will control all aspects of the operation of the test equipment.

[0041] Usually, an RF signal of RF wave has an amplitude, a frequency and a phase. In the current application, a highly precise clock signal is used for stabilizing the RF signal. In fact, an atomic clock is currently used for stabilizing the RF signal. In an embodiment, the phase of the RF signal is stabilized or synchronized with a clock signal. A stable, accurate and affordable clock signal can be obtained from an atomic clock.

[0042] Detailed description of the invention

[0043] Further embodiments of the method and displacement sensing device are discussed below.

[0044] In an embodiment, the method further comprises receiving a clock signal having an accuracy of at least 10'10-th of a second, in particular at least 10-11-th of a second, more in particular from an atomic clock. Furthermore, in this embodiment the method comprises receiving a radio-frequency signal. A radio-frequency of said radiofrequency signal is phase-synchronized to the clock signal for providing said phase- synchronized radio-frequency signal. Next, the phase-synchronized radio-frequency signal is transmitted into a radio-frequency resonance cavity with one end coupled to the first object and a second end coupled to the second object having a cavity length corresponding to the distance. A phase shift of the radio-frequency interference signal with respect to the phase-synchronized radio-frequency signal. Next, from the phase shift the cavity length is determined. Usually, the value is calculated to present a measured displacement. In an embodiment, the radio-frequency resonance cavity comprises an inductor and a capacitor having a capacitance provided by the length of the radio-frequency resonance cavity, wherein a position of a radio-frequency resonance peak is detected by measuring the phase of the radio-frequency interference signal from the resonance cavity comparing it to the phase of the phase-synchronized radio-frequency signal.

[0045] In an embodiment, the resonance of the radio-frequency resonance cavity is set by the inductance of the inductor and the capacitance of the capacitor, wherein the capacitor is provided by a parallel plate capacitor, in particular comprising of two parallel printed circuit boards (PCBs), with one plate connected to the sensing first object and the other plate connected to the second object, wherein the two parallel plates and the air between them define a capacitance that changes, in particular linearly, with a change of the distance between the first object and the second object.

[0046] In an embodiment, the radio-frequency resonance position shifts as the first and second object displace with respect to one another, wherein the radio-frequency phase interferometer compares the phase of the reflected radio-frequency signal and the input radio-frequency signal and converts the phase shift of the reflected radio-frequency signal with respect to the input radio-frequency signal into a displacement.

[0047] In an embodiment, a measurement range is up to 10 mm, in particular up to 100 mm, more in particular up to 1 m, in particular at an accuracy of 1-10 nanometres.

[0048] In an embodiment, a tuneable radio-frequency source is provided for providing a frequency-tuneable radio-frequency signal, wherein in particular a radio-frequency resonance peak position is set via tuning of said frequency-tuneable radio-frequency signal and the displacement is determined from a displacement of said resonance peak.

[0049] In an embodiment of the displacement sensing device, it further comprises

[0050] - a radio frequency transmitter operationally coupled to said radio-frequency signal generator to receive the clock-stabilized radio frequency signal and to send the clock- stabilized radio frequency signal into said electromagnetic resonance cavity, in particular a radio-frequency resonance cavity;

[0051] - a radio frequency reflector attached to the second object and positioned to reflect a radio-frequency signal received from the radio-frequency transmitter back to the radiofrequency transmitter. In an embodiment, the radio-frequency transmitter and the radio-frequency reflector define a radio-frequency resonance cavity between them that has a cavity length proportionally to the distance between the first object and the second object.

[0052] In an embodiment, the radio-frequency transmitter is adapted to send and receive a radio-frequency signal having frequency of between 20 kHz and 300 GHz.

[0053] In an embodiment, the clock signal originates from an atomic clock having an accuracy of at least 10'11of a second, allowing measuring the displacement with an accuracy of at least 10 nanometres. In a particular embodiment the accuracy is at least 1 nanometres. More in particular, the accuracy is at least 0.1 nanometres. In an embodiment, the displacement sensing device further comprises an atomic clock having an accuracy of at least 10'11of a second functionally and coupled to said clocksignal input, allowing measuring the distance with an accuracy of at least 10 nanometres

[0054] In an embodiment, the radio-frequency generator is frequency tuneable, adapted for matching the resonance of the RF cavity. In an embodiment the radio-frequency generator is adapted for matching it up to the RF cavity's full-width half maximum, in particular adapted for providing a homodyne interferometer with the radio-frequency generator tuneable to match the resonance of the RF cavity, in particular up to the RF cavity’s full-width half maximum.

[0055] In an embodiment, the radio frequency generator is adapted for generating the radio frequency source signal having a frequency of between 200 MHz and 20 GHz, more in particular between 300 MHz and 20 GHz, in particular between 1 GHz and 20 GHz.

[0056] In an embodiment, the displacement sensing device comprises a radio-frequency circuit, a demodulation stage and low pass filtering stage in operation providing an output signal, said output signal providing a DC voltage which changes with the resonance cavity length.

[0057] In an embodiment, the displacement sensing device comprises a phase-locked loop (PLL) comprising at a PLL input operationally coupled to the radio-frequency generator and to the clock signal input and comprising a PLL output providing the clock-stabilized radio-frequency signal. In a particular embodiment, the phase-locked loop comprises a digital phase-locked loop, with said PLL output coupled to said electromagnetic resonance cavity. In an embodiment, the radio-frequency interferometer comprises a phase detector downstream coupled to a low pass filter.

[0058] A new method is presented that is suitable for to measure and stabilize for instance a platform position with nanometre accuracy, which can be utilized for developing a nano positioning sensor. Measurement speed is fast, in fact a speed of between 10 kHz and 1 MHz is possible.

[0059] A nano positioning system consists of a multi-axis translation stage with a resolution of a few tens of nanometres or less, along with a positioning sensor that measures position and provides feedback. This feedback system ensures that the stage remains at the desired position without any drift or noise. This technology is widely used in various fields such as microscopy, where it can be used to move the sample, and in the semiconductor industry for wafer translations.

[0060] Nano positioning sensors are the equipment that enable ultra-precise positioning of objects. These systems are used in various applications, including but not limited to microscopy, semiconductor manufacturing, optical alignment, and bionanotechnology. With the help of nano positioning systems, accurate positioning can be achieved together with high precision.

[0061] In another application, the system can be uses to maintain a distance between two objects. This can also result in maintain a distance between two reflectors, thus maintaining a length of a resonance cavity. Such a cavity may for instance comprise an optical resonance cavity.

[0062] To detect minute changes in the relative positioning of two platforms, a method is used that involves an RF resonator and a stable RF source. The RF resonator is provided using a custom-made inductor and capacitor. The position of the RF resonance peak is detected with extreme precision by measuring the phase of the reflected RF signal from the resonator and comparing it to the input phase. Thus, in fact, the accuracy of a clock signal, i.e. time, is transferred to another SI quantity, like length. This uses the extreme accuracy at which the speed of light (or electromagnetic radiation) is known.

[0063] The resonance of the RF cavity is determined by the inductor and capacitor used. In the current proposal, a custom parallel plate capacitor design is used. This comprises two parallel printed circuit boards (PCBs) - one carrying a sensing platform and the other on holding a reference platform. The two parallel plates or circuit board surfaces and the air between them create a capacitance that changes proportionally as the relative position of the reference platform to the sensing platform changes. If both parallel plates are closely together, for instance closer than a wavelength of the RF signal, the response will be in the nearfield regime, which is not linear but is still proportional. Thus, selecting a shorter RF wavelength allows for measuring displacement when parallel plates are close together, usually several wavelengths.

[0064] The position of the RF resonance is coupled to the relative positioning of the two platforms or circuit boards. As the sensing platform moves with respect to the reference platform, the RF resonance position shifts, which is detected by a RF phase interferometer. This RF phase interferometer compares the phase of the reflected RF wave (which travelled a different path then the input RF wave) with respect to the phase of the input RF wave and converts it into a position change. This allows measurement of the relative position change with great accuracy and allows to calibrate it to the RF source.

[0065] The current method and system can introduce a new class of nano positioning sensors to the market of nano positioning systems. Capacitance sensors are the closest type of sensors that can perform nanometre position sensing. However, our sensing method can complement this sensor with better range, accuracy and precision in both short and long-time scales. Our method derives its accuracy and precision from the RF source, where we translate the precision and accuracy of an RF frequency to position sensing by using it as a ruler for position. The accuracy and precision of the RF signal can be linked to an atomic clock signal, like the current GPS signal, and can perform up to 1 part in a trillion times. The performance of a capacitance sensor is limited to the performance of the reference capacitor, which needs individual calibration and still would drift with environmental changes. In contrast, an RF source referenced to an atomic clock (like GPS signal) can easily be better than a capacitance reference by several orders of magnitude.

[0066] The terms “upstream” and “downstream” relate to an arrangement of items or features relative to the propagation of the light from a light generating means (here the especially the first light source), wherein relative to a first position within a beam of light from the light generating means, a second position in the beam of light closer to the light generating means is “upstream”, and a third position within the beam of light further away from the light generating means is “downstream”. The term “substantially” herein, such as in “substantially consists”, will be understood by the person skilled in the art. The term “substantially” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective substantially may also be removed. Where applicable, the term “substantially” may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. The term “comprise” includes also embodiments wherein the term “comprises” means “consists of’.

[0067] The term "functionally" will be understood by, and be clear to, a person skilled in the art. The term “substantially” as well as “functionally” may also include embodiments with “entirely”, “completely”, “all”, etc. Hence, in embodiments the adjective functionally may also be removed. When used, for instance in “functionally parallel”, a skilled person will understand that the adjective “functionally” includes the term substantially as explained above. Functionally in particular is to be understood to include a configuration of features that allows these features to function as if the adjective “functionally” was not present. The term “functionally” is intended to cover variations in the feature to which it refers, and which variations are such that in the functional use of the feature, possibly in combination with other features it relates to in the invention, that combination of features is able to operate or function. For instance, if an antenna is functionally coupled or functionally connected to a communication device, received electromagnetic signals that are receives by the antenna can be used by the communication device. The word “functionally” as for instance used in “functionally parallel” is used to cover exactly parallel, but also the embodiments that are covered by the word “substantially” explained above. For instance, “functionally parallel” relates to embodiments that in operation function as if the parts are for instance parallel. This covers embodiments for which it is clear to a skilled person that it operates within its intended field of use as if it were parallel.

[0068] Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. The devices or apparatus herein are amongst others described during operation. As will be clear to the person skilled in the art, the invention is not limited to methods of operation or devices in operation.

[0069] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements. In the device or apparatus claims enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0070] The invention further applies to an apparatus or device comprising one or more of the characterising features described in the description and / or shown in the attached drawings. The invention further pertains to a method or process comprising one or more of the characterising features described in the description and / or shown in the attached drawings.

[0071] The various aspects discussed in this patent can be combined in order to provide additional advantages. Furthermore, some of the features can form the basis for one or more divisional applications.

[0072] Brief description of the drawings

[0073] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0074] Figure 1 schematically depicts an embodiment of a displacement sensor;

[0075] Figure 2 shows measurements of the displacement sensor of figure 1, and

[0076] Figures 3, 4 show a stability example

[0077] The drawings are not necessarily on scale. Description of preferred embodiments

[0078] Figure 1 schematically depicts an embodiment of a displacement sensor. The displacement sensor allows an accurate measurement of an amount of displacement of a first object and a second object with respect to one another. The illustrated displacement sensing device or displacement sensor requires a stable reference time signal. In the current embodiment, a clock signal is provided, here originating from an atomic clock 1. In fact, any clock signal with an accuracy of better than 10'10-th part of a second, in particular 101th part of a second, might be suitable. A clock accuracy of 10-11th part of a second may allow an accuracy of 10 nanometre.

[0079] The clock signal is received on a clock signal input and is provided as input to a programmable clock generator 2. This programmable clock generator 2 outputs a clock-stabilized RF signal. In an alternative embodiment, for instance a phase locked loop (PLL) may be used for synchronizing the phase of the RF signal. In such an embodiment, an RF generator with a frequency setting may be used.

[0080] The displacement sensing device or displacement sensor further comprises an RF splitter 3. An input of the RF splitter 3 is coupled to receive the clock-stabilized RF signal. The RF splitter 3 results in two output signals, first a continuing signal, indicated as “ Input”. That RF signal enters or is coupled to an RF coupler 4. The RF coupler 4 couples the RF signal to a transmitter 5. Opposite the transmitter 5, an RF reflector 6 is provided. A distance d between the transmitter 5 (“ mirror 1”) and reflector 6 (“mirror 2”) provides an RF cavity or RF resonance cavity with length d. The RF resonance cavity in fact provides a capacitor with a capacitance which relates to the length d. In order to provides measurement of a displacement between a first object 10 or reference platform 10 and a second object 11 or target platform 11, the transmitter 5 is mounted upon the first object 10 and the RF reflector 6 is mounted upon or onto the second object 11.

[0081] The illustrated distance sensing device further comprises an RF Phase detector 7. The RF phase detector 7 receives the interference signal from the RF cavity via the coupler 4, and receives as input an RF signal from the RF generator 2, the clock- stabilized RF signal. The output of the phase detector 7 is subsequently input to an Analogue to digital converter (ADC) 8. The signal from the ADC 8 is input to a data acquisition system 9. In principle, a homodyne interferometer using a highly stable RF source is provided. Using the clock signal, the phase of the RF signal is linked to the (time) stable clock signal.

[0082] In an embodiment, the source frequency of the RF source signal is tuneable to match a resonance of an RF resonance cavity's, in fact match it up to a full-width half maximum. Stabilization is done here using an atomic clock signal. An RF circuit, along with a demodulation stage and low pass filtering stage provides an output signal. This output signal comprises a DC voltage that changes with the relative position of a sensing and reference PCB comprising the RF transmitter.

[0083] In an embodiment, a phase-locked-loop (PLL) system creates a programmable clock output signal from atomic clock output (for instance a 10 MHz clock). This provides as an output signal an RF signal from the programmable clock generator that is phase locked with the atomic clock signal. This in fact translates the stability of atomic clock signal to the arbitrary RF frequency that is used for position sensing.

[0084] A PLL system in general comprises four blocks: a phase detector, a low pass filter, a voltage controlled oscillator (VCO) and a feedback system.

[0085] The phase detector compares the phase of the input signal (reference clock signal) with the phase of the feedback signal from the loop's output. The output from the phase detector is an analogue voltage that is proportional to the phase difference between these two signals. The output of the phase detector, which typically contains high-frequency noise and the desired phase error signal, is fed into a low-pass filter. The purpose of this filter is to remove the high-frequency components and pass a smooth error signal (DC level) that is proportional to the phase error. The filtered output from the loop filter is used to control the voltage-controlled oscillator (VCO). The VCO generates an output frequency that is proportional to its input voltage. Changes in the input voltage, reflecting changes needed to correct phase errors, adjust the frequency of the VCO. The VCO's frequency is thus continually adjusted to match the phase of the reference signal as closely as possible to get the best phase and frequency conversion from the clock frequency to the desired, set output frequency. The output of the VCO is fed back as one of the inputs to the phase detector. However, before it reaches the phase detector, it often passes through a frequency divider, which reduces the frequency of the VCO output to a level that can be compared with the reference signal. This division allows the PLL to lock to frequencies that are a multiple of the reference frequency.

[0086] In an experimental displacement sensor, a AD9858 direct digital synthesizer chip obtained from Analogue Devices was used to perform this step.

[0087] Using the experimental set-up, it proved possible to detect a displacement of less than 5 nm with this prototype. The prototype or experimental set-up has room for further improvements and may be improved to allow a displacement measurement resolution and accuracy of at least one order, in particular two orders of magnitude in performance increase.

[0088] A improvement in an embodiment may provide a reduction in the size of the displacement sensor read-out RF electronic and reduces or even eliminates electronic noise sources. The displacement sensor including 90 % of its read-out electronics can for instance be accommodated into a printed circuit board (PCB) having a size of 25 mm by 25 mm. A version thus upgraded should provide a displacement sensor that for instance serves its purpose in the lab for a microscope translation stage.

[0089] Further, the measurement range of that sensor can be increased from 1 mm to 100 mm. This would open up new areas where this sensor can be employed like machining and manufacturing industry and 3D printers.

[0090] Experimental I

[0091] The results from the current prototype already support the invention, as illustrated in figure 2. Figure 2 depicts a plot or graph showing the current (experimental) displacement sensor output against the distance between the first and second object measured using a reference device, here a Michelson interferometer. To illustrate that displacements of a displacement stage can be sensed or measured with nanometre (nm) resolution and accuracy, the graph shows the current displacement sensor output that changes for such small displacements and at the same time use another reference method to measure the displacements with even better resolution. Now if the current method or experimental displacement sensor can already achieve this resolution and low noise level to detect displacement.

[0092] Figure 2 shows that the current displacement sensor / device can measure a change of position of the positioning stage with a resolution of better than 10 nm. To demonstrate this high resolution, an inset plot is provided which is a zoomed part of the main graph. In the insert, since the whole plot range is 50 nm, it shows that the method senses displacement's to less than 10 nm in the current prototype version. The frequency of the laser used for the reference device which is the Michelson interferometer is referenced to an 87Rb transition in an atomic vapor cell with < 1 MHz precision. The measurement gap is ~ 2 mm. The current displacement sensor operates at an operating RF frequency of 340 MHz. Using the test embodiment, a resolution of better than 10 nm was attained in this configuration. The nonlinearity in the data from the fit, in fact shows the nonlinearity of the displacement stage and not from the sensor! This means the deviation of blue data points from red lines is not from the sensor, it is from positioning stage.

[0093] Please note:

[0094] The red line shows a linear fit through the blue measurements.

[0095] The red dots depict a 2c (2-sigma) standard deviation as obtained from a linear fit to the data, where 2c gives the usual confidence interval of 95 %. For readability, the fitted line and 2c indications (red dots) are removed from the insert plot.

[0096] The blue line represents the data points. Since the number of data points are very high it appears to be a line.

[0097] The error (red dots) is derived by fitting the blue data points to a linear curve which is represented as the red dotted line.

[0098] The data above illustrates that using an improved sensor, it is possible to increase accuracy up to 1 nm / meter! In other words, a displacement of 1 nm can be measured at a separation of 1 meter.

[0099] Experimental II

[0100] In figures 3 and 4, examples are shown indicating the stability of the currently proposed sensor and method.

[0101] In figure 3, we measure the distance between the reference plane and object plane using an independent reference laser interferometer at 780 nm laser as an industrial and research standard. The laser of this laser interferometer is actively stabilized to a Rubidium atomic transition for absolute stability. In particular a two- port Michelson interferometer was used. In the locked case, we use our technology to sense the distance between the reference plane and object plane and perform an active stabilization (locked, red) on the position of object plane with respect to reference plane. The active stabilization is implemented using a standard piezo actuator that controls the position of the object plane. In particular, the piezo actuator is of the type of rectangular stacked design (Thorlabs PK4FMYP2) and has an resolution of < 0.1 nm (Including the noise of the voltage source used to drive the piezo actuator).

[0102] A setup for the current sensor is provided, in which the atomic clock is of the type Rb atomic clock from Standford Research Systems and has an accuracy of IO'10fractional stability at 1 Second. The laser used is of the type Toptica DLPro at 780 nm and is referenced to a Rb atomic transition. This laser offers a long term fractional frequency stability of < 10'9.

[0103] Further components used are FPGA based PID controller and analog front end electronic for stabilizing the position.

[0104] In the unlocked case (blue), there is no active stabilization engaged. The above data shows a clear improvement of position stability over the duration of 90 seconds in the actively stabilized case using our current sensor.

[0105] In figure 4, we show an histogram of the data for figure 3.

[0106] We observe the position distribution in the case of actively stabilizing using our sensor reduces the position uncertainty to ~2.5 nm (standard deviation (one sigma) of the red distribution). In fact, this also includes an accuracy of the reference laser interferometer of ~2 nm (one sigma). The limited accuracy of the interferometer is due to the environmental fluctuation such as temperature and pressure variations.

[0107] It will also be clear that the above description and drawings are included to illustrate some embodiments of the invention, and not to limit the scope of protection. Starting from this disclosure, many more embodiments will be evident to a skilled person. These embodiments are within the scope of protection and the essence of this invention and are obvious combinations of prior art techniques and the disclosure of this patent. Reference numbers

[0108] 1 Atomic clock

[0109] 2 Programmable clock generator 3 RF splitter

[0110] 4 RF coupler

[0111] 5 RF reference transmitter (Mirror 1)

[0112] 6 RF target reflector (Mirror 2)

[0113] 7 RF Phase detector 8 Analog to digital converter

[0114] 9 Data acquisition system

[0115] 10 Reference platform

[0116] 11 Target platform d object distance

Claims

Claims1. A method for establishing an amount of displacement of a first and a second object with respect to one another, comprising:- generating a phase-synchronised radio-frequency signal which is signal phase- synchronised using a clock signal having an accuracy of at least 10'10-th of a second, in particular at least 10’n-th of a second, more in particular at least 10'12-th of a second;- transmitting said phase-synchronised radio-frequency signal into a resonance cavity having the first and second object coupled to opposite ends of said cavity;- extracting an electromagnetic cavity signal from an end of said cavity;- extracting an radio-frequency interference signal from said electromagnetic cavity signal, and- determining a cavity length from the radio-frequency interference signal, and determining from said cavity length said amount of displacement.

2. The method of claim 1, further comprising receiving a clock signal having an accuracy of at least 10'10-th of a second, in particular at least 10-11-th of a second, more in particular from an atomic clock, receiving a radio-frequency signal, phasesynchronizing a radio-frequency of said radio-frequency signal to the clock signal for providing said phase-synchronized radio-frequency signal, transmitting the phase-synchronized radio-frequency signal into a radio-frequency resonance cavity with one end coupled to the first object and a second end coupled to the second object having a cavity length corresponding to the distance, determining a phase shift of the radio-frequency interference signal with respect to the phase- synchronized radio-frequency signal, and determining from the phase shift the cavity length.

3. The method of claim 2, wherein the radio-frequency resonance cavity comprises an inductor and a capacitor having a capacitance provided by the length of the radiofrequency resonance cavity, wherein a position of a radio-frequency resonance peak is detected by measuring the phase of the radio-frequency interference signalfrom the resonance cavity comparing it to the phase of the phase-synchronized radio-frequency signal.

4. The method of claim 2 or 3, wherein the resonance of the radio-frequency resonance cavity is set by the inductance of the inductor and the capacitance of the capacitor, wherein the capacitor is provided by a parallel plate capacitor, in particular comprising of two parallel printed circuit boards (PCBs), with one plate connected to the sensing first object and the other plate connected to the second object, wherein the two parallel plates and the air between them define a capacitance that changes, in particular linearly, with a change of the distance between the first object and the second object.

5. The method of claim 4, wherein the radio-frequency resonance position shifts as the first and second object displace with respect to one another, wherein the radiofrequency phase interferometer compares the phase of the reflected radiofrequency signal and the input radio-frequency signal and converts the phase shift of the reflected radio-frequency signal with respect to the input radio-frequency signal into a displacement.

6. The method of any one of the preceding claims, wherein a measurement range is up to 10 mm, in particular up to 100 mm, more in particular up to 1 m, in particular at an accuracy of 1-10 nanometres.

7. The method of any one of the preceding claims, wherein a tuneable radiofrequency source is provided for providing a frequency-tuneable radio-frequency signal, wherein in particular a radio-frequency resonance peak position is set via tuning of said frequency-tuneable radio-frequency signal and the displacement is determined from a displacement of said resonance peak.

8. A displacement sensing device for establishing an amount of displacement from a first and a second object with respect to one another, comprising:- a clock signal input for providing a clock signal to the displacement sensing device, said clock signal having an accuracy of at least 10'10of a second, inparticular at least 10'11of a second;- a radio-frequency signal generator for providing a radio-frequency source signal, the radio-frequency generator phase coupled to the clock signal input in a synchronized manner for generating a phase-stabilized radio-frequency signal;- an electromagnetic resonance cavity electromagnetically coupled to the radiofrequency generator, said electromagnetic resonance cavity having one cavity end coupled to the first object and a second cavity end coupled to the second object;- a radio-frequency interferometer functionally coupled to the clock-stabilized radio-frequency signal and to the reflected radio-frequency signal for providing an output signal that is proportional to the displacement.

9. The displacement sensing device of claim 8, further comprising- a radio frequency transmitter operationally coupled to said radio-frequency signal generator to receive the clock-stabilized radio frequency signal and to send the clock-stabilized radio frequency signal into said electromagnetic resonance cavity, in particular a radio-frequency resonance cavity;- a radio frequency reflector attached to the second object and positioned to reflect a radio-frequency signal received from the radio-frequency transmitter back to the radio-frequency transmitter.

10. The displacement sensing device of claim 8 or 9, wherein the radio-frequency transmitter and the radio-frequency reflector define a radio-frequency resonance cavity between them that has a cavity length proportionally to the distance between the first object and the second object.

11. The displacement sensing device of claims 8-10, wherein the radio-frequency transmitter is adapted to send and receive a radio-frequency signal having frequency of between 20 kHz and 300 GHz.

12. The displacement sensing device any one of claims 8-11, wherein said clock signal originates from an atomic clock having an accuracy of at least 10'11of a second, allowing measuring the displacement with an accuracy of at least 10 nanometres,in particular at least 1 nanometres, more in particular at least 0.1 nanometres.

13. The displacement sensing device of any one of the preceding claims 8-12, further comprising an atomic clock having an accuracy of at least 10'11of a second functionally and coupled to said clock-signal input, allowing measuring the distance with an accuracy of at least 10 nanometres14. The displacement sensing device of claims 8-13, wherein the radio-frequency generator is frequency tuneable, adapted for matching the resonance of the RF cavity, in particular matching it up to the RF cavity's full-width half maximum, in particular adapted for providing a homodyne interferometer with the radiofrequency generator tuneable to match the resonance of the RF cavity, in particular up to the RF cavity’s full-width half maximum.

15. The displacement sensing device of any one of the preceding claims 8-14, wherein the radio frequency generator is adapted for generating the radio frequency source signal having a frequency of between 200 MHz and 20 GHz, more in particular between 300 MHz and 20 GHz, in particular between 1 GHz and 20 GHz.

16. The displacement sensing device of any one of the preceding claims 8-15, wherein said displacement sensing device comprises a radio-frequency circuit, a demodulation stage and low pass filtering stage in operation providing an output signal, said output signal providing a DC voltage which changes with the resonance cavity length.

17. The displacement sensing device of any one of the preceding claims 8-16, comprising a phase-locked loop (PLL) comprising at a PLL input operationally coupled to the radio-frequency generator and to the clock signal input and comprising a PLL output providing the clock-stabilized radio-frequency signal, in particular the phase-locked loop comprising a digital phase-locked loop, with said PLL output coupled to said electromagnetic resonance cavity.

18. The displacement sensing device of any one of the preceding claims, wherein the radio-frequency interferometer comprises a phase detector downstream coupled to a low pass filter. -o-o-o-o-o-

Images