Patent Application
20080013102
The present invention relates to a sensor device comprising at least one sensor element with a plurality of sensitive layers, in particular to a micro-electromechanical sensor (MEMS) device with at least one sensor element being deflectable in response to a coupling of a probe substance to a sensitive layer of the sensor element. Furthermore, the present invention relates to a measuring device for sensing probe substances, which measuring device includes the above sensor device. Furthermore, the present invention relates to a method of investigating a sample for sensing at least one probe molecule using the above sensor and measuring devices. The invention can be used for sensing probe substances in physics, chemistry, biology and applied techniques.
The application of MEMS devices is generally known in particular in the field of molecular biology for sensing or investigating biologically relevant molecules. Typically, MEMS devices include a cantilever element carrying a sensitive layer. Mechanical or geometric properties of the cantilever are changed in response to an interaction of a probe substance with the sensitive layer. Basic features of MEMS devices are described e.g. by R. Raiteri et al. in “Materials Today”, vol. 5, Jan. 2002, p. 22-29.
A first operation principle of MEMS devices is based on measuring a bending or deflection of the cantilever in response to a surface stress caused by the interaction of the probe substance with the sensitive layer (see e. g. L. A. Pinnaduwage et al. in “Sensors and actuators”, vol. B99, 2004, p. 223-229). The deflection of the cantilever is optically measured e.g. with a beam deflection technique. The measurement of cantilever deflection has an essential advantage in terms of increased sensitivity. Even a few molecules of a probe substance are capable to induce the surface stress and bending of the cantilever. Furthermore, as the deflection method is a static technique, the substance sensing can be applied without restrictions in vapours as well as in liquids. However, there is a disadvantage as simultaneous quantitative sensing of different molecules is difficult with the conventional deflection measurement.
For simultaneous investigating a plurality of different molecules, the use of cantilever arrays has been proposed (see e.g. S. Yeon et al. in “Applied Physics Letters”, vol. 85, 2004, p. 1083-1084). The cantilever array comprises a plurality of cantilevers fixed to a common holding body. Each cantilever carries one sensitive layer with a specific sensitivity for a particular probe substance. If a so-called pattern recognition technique is applied for analyzing the cantilever bending, the number of substances detected with the cantilever array can be larger than the number of cantilevers. However, the number of cantilevers forming one common array is restricted due to the available micromachining techniques. Furthermore, while cantilever arrays can be produced with increasing numbers of cantilevers (e.g. more than 100), the optical beam deflection measurement of the cantilever bending would require multiple measurements with multiple sensors for obtaining a sufficient precision and reproducibility of substance sensing.
Furthermore, 2-dimensional cantilever arrays have been described (see e.g. M. Yue et al. in “Journal of Micro-electromechanical Systems”, vol. 13, 2004, p. 290-299). 2-dimensional cantilever arrays have disadvantages in terms of the complex micro-mechanical manufacturing of 2-dimensional rows of sensors and optical detection of the cantilever bending.
M. Helm et al. have proposed an improved optical measurement for cantilever-based sensors, wherein interferometry was applied for measuring cantilever bending (“Applied Physics Letters” vol. 87, 2005, p. 064101). A single cantilever sensor was arranged in one arm of a phase shift interferometer. An interference pattern on the cantilever surface was measured with a CCD camera and evaluated for calculating the curvature of the cantilever in response to coupling of a probe substance. The application of the phase shifting interferometry by M. Helm et al. represents an important advantage for high sensitivity deflection measurements. M. Helm et al. have measured deflections smaller than 2 nm. However, the conventional interferometry technique was restricted to the measurement in air with one single cantilever so that a simultaneously measurement of a plurality of substances was impossible.
Another operation principle of conventional cantilever-based sensors comprises a dynamical measurement of a resonance frequency of the cantilever, which is changed in dependence on a mass loading at the apex of the cantilever. The operation of the cantilever as a precise balance is given for substances sensing in a gaseous atmosphere. This restriction represents an essential disadvantage in particular for applications in biology, as the biologically relevant molecules are typically available in liquids. For obtaining a sufficient precision in substance sensing, the dynamical measurement requires a high resonance frequency (typically 100 kHz or higher) of the cantilever. Accordingly, a cantilever has to be used having a high spring constant.
U.S. Pat. No. 5,719,324 discloses a MEMS device with a cantilever sensor element carrying a sensor array with up to three layers being sensitive for different probe substances. The sensitive layers are arranged along the length of the cantilever at particular positions selected for a response at the fundamental resonance frequency, the second harmonic or third harmonic, respectively. Mass loading at the different sensitive layers is measured by exciting the fundamental and harmonic frequencies and monitoring the damping of the cantilever at the various frequencies.
Due to the following essential disadvantages, the technique according to U.S. Pat. No. 5,719,324 has not found a practical application. Firstly, the operation of the MEMS device is limited to a gaseous atmosphere. Furthermore, the resonance properties of the cantilever are read-out sequentially with the conventional technique. Accordingly, a real multiplex operation with parallel measurements is impossible. Furthermore, the contributions of the different sensitive layers cannot be measured separately. The cross-talk of the sensitive layers represents a serious limitation for the specific detection of different molecules. In particular, the sensitive layer of the array adjacent to the holding body provides a contribution in all frequency ranges.
All conventional techniques for probe substance sensing have an essential disadvantage with regard to the restricted multiplex capability. In practice, there is an interest in exposing a plurality of sensitive layers (more than 100 or even more than 1.000) to an analyte for simultaneous sensing a plurality of different molecules.
The object of the invention is to provide an improved sensor device avoiding the disadvantages of the conventional techniques and being capable to sense a plurality of probe substances, wherein the sensor device allows the simultaneous detection of an increased number of probe substances with high sensitivity and reproducibility. In particular, the object of the invention is the provision of the sensor device including a plurality of sensitive layers, which can be read out with reduced cross-talk. Furthermore, the object of the invention is the provision of an improved measuring device avoiding the disadvantages of the conventional operation principles for reading-out sensor elements and being capable for an improved detection of a sensor element deflection with increased precision and reproducibility. Furthermore, the object of the invention is to provide an improved method of investigating an analyte sample for sensing at least one probe molecule in the sample.
These objects are solved with sensor and measuring devices and methods comprising the features of claims 1, 19 or 25, respectively. Advantageous embodiments of the invention are defined in the dependent claims.
According to a first aspect, the present invention is based on the general technical teaching of providing a sensor device with at least one sensor element carrying a plurality of sensitive layers each of which being capable for a physical of chemical interaction with at least one probe molecule, wherein, after an interaction-induced change in surface stress at one or more of the sensitive layers, the sensor element is locally deformable. To this end, the material and geometry of the at least one sensor element is selected for providing a spring constant below 5 N/m at least in the direction of bending the sensor element in response to a change in surface stress induced by the respective sensitive layer. The inventors have found that a spring constant below the 5 N/m limit is particularly suitable for high sensitive substance sensing with a sensor element bending along a sensitive layer of about 1 nm.
According to the invention, the local deformation is provided for each portion of the sensor element carrying one of the sensitive layers independently on the surface stress conditions at the remaining portions of the sensor element. The at least one sensor element has a surface contour, which is determined by the stress conditions at each of the sensitive layers.
The group of sensitive layers on one surface of the sensor element provide a sensor array. The inventors have found that the sensitive layers can be deposited on the surface with a pre-selected array pattern. Advantageously, there are no particular limitations with regard to the specific positions of the sensitive layers on the sensor element as the sensor device can be read-out by an optical measurement, in particular by an interferometric measurement with extremely high precision. In particular, the sensitive layer positions do not depend on mechanical vibration properties of the sensor element. Each region of the sensor element carrying one sensitive layer provides an independent sensor unit. The sensor units of one sensor element can be read out independently from each other.
The sensor device of the invention has another important advantage in that the complete surface of the at least one sensor element can be read-out with one single optical measurement only. Due to the local and specific bending of the sensor element at the positions of the sensitive layers, the conventional operation principle with the excitation of a series of vibration frequencies can be avoided. The sensor device of the present invention offers for the first time a parallel measurement at a plurality of sensitive layers arranged on one single sensor element, like a single micro-mechanical cantilever or membrane sensor element.
Manufacturing a micro-mechanical sensor device according to the invention comprises a deposition of a plurality of sensitive layers on a surface of a single micro-mechanical sensor element at positions, which are selected independently of vibration properties of the sensor element and in particular independently of the positions of nodes or antinodes of higher harmonics of sensor element vibrations.
Advantageously, the sensor device according to the invention allows a simple adaptation to an increased number of substances to be detected. An increased number of substances can be detected without increasing the number of sensor devices but rather with one single sensor device of the invention equipped with a plurality of sensitive layers.
Advantageously, there are no limitations with regard to the geometric arrangement of the sensitive layers relative to each other. If the sensitive layers are arranged as a one-dimensional series, e.g. as a straight row, particular advantages can be obtained in terms of using conventional micro-mechanical cantilever structures being adapted for a static read-out of the cantilever deflection. Alternatively, the sensor array may comprise a two-dimensional arrangement of sensitive layers, which advantageously increases the number of parallel readable local deflections.
In contrast to the conventional MEMS device with a sensor array, the sensor device of the invention can be operated under various environmental conditions, in particular in gases or liquids. According to a preferred embodiment of the invention, the sensor device comprises a calibration structure, which is carried by a component of the sensor device, like e.g. a holding body or the at least one sensor element. The calibration structure is a solid component with a predetermined three-dimensional geometrical structure, like e.g. a step. The provision of the calibration structure allows a compensation of variations in the refractive index with changing environments of the sensor element, like with exchanging of liquids or gases or with a changing illumination angle during interferometric deflection measurement. Preferably, the calibration structure is integrated on a micro-mechanical chip providing the sensor device. It facilitates the imaging interferometry for reading-out the sensor element deflections. Furthermore, the calibration structure facilitates the adjustment of the sensor device in a measuring device for optically reading the sensor element deflections.
If, according to a further preferred embodiment of the invention, the sensor device comprises a deflection device being capable to compensate local deformations of the at least one sensor element, the sensitivity of the sensor device can be essentially increased. Sensor elements, like cantilever sensor elements or membrane sensor elements have a maximum sensitivity, if they are not deflected or bent. The deflection device is arranged for compensating each local surface stress of the sensor element so that the sensor element is not bent even with an interaction with one or more probe substances. Advantageously, any operation parameter of the deflection device like e.g. an electrical current for locally operating the deflection device can be used for reading-out the sensor device of the invention.
Preferably, the deflection device is operated on the basis of at least one of the following techniques. With a heating deflection device, a local heating of the sensor element is generated for compensating the local surface stress. With a magnetic field deflection device, any surface bending of the sensor element can be compensated with locally effective magnetic fields. The corresponding effect can be obtained with an electrostatic field deflection device. Mechanic deflection compensation can be provided with an actuator deflection device or a pressure deflection device, which are capable of subjecting the sensor element to a local mechanical driving force.
Another advantage of the invention is given by the variability in fixing the at least one sensor element to a holding body, like e.g. a chip body of a micro-mechanical frame. According to a first variant, the at least one sensor element or preferably a plurality of sensor elements are fixed only with one base portion to the holding body. Advantageously, conventional multi cantilever chips can be used for manufacturing such a sensor device. According to a second variant, the at least one sensor elements or preferably a plurality of sensor elements are fixed with at least two side portions to the holding body. Preferably, the sensor element is fixed with side portions opposite to each other. With this embodiment, the holding body has a frame shape wherein the sensor elements are connected with opposite parts of the frame. According to a third variant, the holding body is a compact carrier block and the at least one sensor element is sandwiched on a surface of the carrier block. The carrier block has through-holes for providing a space, into which the sensor element can be bent. This embodiment has particular advantages in terms of stability and compact structure.
Further preferred embodiments of the invention are related to various shapes and arrangements of the at least one sensor element. Firstly, the sensor element can be provided with a strip shape, i.e. with a rectangular surface, the length of which being essentially larger than the width. The strip shape has an aspect ratio length:width of e.g. 100 μm to 10 μm. Preferably, a plurality of strip-shaped sensor elements are fixed to the holding body. Alternatively, the at least one sensor element has a membrane shape, i.e. a rectangular shape with similar length and width dimensions.
According to a particularly preferred embodiment of the invention, the at least one sensor element comprises at least one support structure, which is adapted for stabilising the sensor element with regard to at least one direction. Each support structure comprises a portion of the sensor element with an increased thickness. Preferably, the support structure comprises a web profile extending perpendicular to the longitudinal extension of the sensor element, e.g. the cantilever sensor element. Advantageously, with the support structure, the sensor element cannot be deflected perpendicular to the longitudinal extension thereof. Plane portions with a predetermined spring constant are formed between the support structures. Preferably, the sensor element carries the sensitive layers between the support structures.
For creating a predetermined environment, the sensor device is arranged in a sample chamber. According to a preferred embodiment of the invention, at least one wall of the sample chamber includes at least one transparent portion. Preferably, the sample chamber wall includes a transparent window. With this embodiment, the optical read-out of the sensor device is facilitated. According to a further preferred embodiment of the invention, the sample chamber forms a liquid-tight enclosure, so that the sensor device can be operated in a liquid environment. The sample chamber can be filled with a liquid being the analyte sample.
If at least one wall of the sample chamber is formed by the sensor device, advantages in terms of a compact and robust structure are obtained. Preferably, the holding body carrying the at least one sensor element forms one wall of the chamber.
As the sensor device is adapted for a static deflection measurement, there are no limitations with regard to the material of the at least one sensor element. In particular, the sensor element can be made of a semiconductor material, a ceramic, a polymer material or a combination thereof.
According to a particularly preferred embodiment of the invention, the at least one sensor element carries a sensor array with at least four, particularly preferred at least eight sensitive layers. With practical conditions, sensing of at least two different molecules is preferred. With two sensitive layers and two reference layers, this objective can be fulfilled with one single sensor element only.
Preferably, the sensor device of the present invention is provided with at least one of the following features. For an improved local bending, the spring constant preferably is below 2 N/m. With the available cantilever materials, the at least one sensor element has a preferred thickness below 2 μm and a preferred length above 400 μm. Advantageously, the size of the single sensitive layers can be selected in a range of 50 μm to 5 μm. The density of the sensor arrays can be increased compared with the conventional techniques.
According to a further advantageous embodiment of the invention, the sensor array comprises sensitive layers arranged on both surfaces of the at least one sensor element. Accordingly, the number of the sensitive layers on the sensor element can be increased. If the sensitive layers are arranged on the surfaces of the sensor element with a partial overlap relative to each other, further advantages can be obtained in terms of providing particular reference regions facilitating the evaluation of the sensor element bending.
According to a second general aspect, the present invention is based on the general technical teaching of providing a measuring device for sensing at least one probe substance, comprising the sensor device according to the invention and an optical profilometer device for optically measuring at least one deflection of the at least one sensor element of the sensor device. Preferably, the profilometer device is adapted for simultaneous measuring (imaging) all local deflections of the at least one sensor element. The inventors have found that available profilometer devices, like e.g. imaging interferometry or confocal microscopy devices, yield curvature patterns (e.g. interferogram or bending patterns) of a surface of the at least one sensor element, which can be evaluated with regard to all local deflections at the positions of the sensitive layers. An imaging profilometer device provides a simultaneous read-out of the local deflections. All sensor units can be measured simultaneously allowing a highly parallel measuring procedure. Advantageously, the local deflections are sensed in absolute quantities (radii of curvature), which allow a direct quantitative determination of the substance concentration.
Advantageously, there are no limitations with regard to the type of profilometer device used for reading-out the curvature patterns. As an example for imaging interferometry, white light interferometry or interferometry with a laser source emitting at one or more single laser wavelengths can be used. Using a laser source with multiple wavelengths has the advantage of detecting the sensor element deflection with increased precision. With confocal microscopy, e.g. a scanning microscope can be used.
If the sensor device comprises the above deflection device for compensating local deflections of the sensor element, the measuring device of the invention, preferably comprises a feedback loop for controlling the deflection device. Advantageously, the feedback loop provides a deflection device control such that each local bending of the sensor element is compensated automatically. Accordingly, the measuring device can be operated with maximum sensitivity.
If, according to a further preferred embodiment of the invention, the profilometer device is adapted for a simultaneous evaluation of the complete curvature pattern, all local deflections of all sensor elements of one sensor device can be obtained simultaneously. With this embodiment, the measurement speed is essentially increased. If additionally profiles, e.g. interferences or focus levels at the above calibration structure are detected, the absolute local deflections of the sensor element can be measured with improved precision. The absolute local deflections of the sensor element can be measured even with an exchange of the environment.
Preferably, the measuring device of the invention is provided with an image processing unit being adapted for calculating the local deflections of the sensor elements from the light curvature patterns measured at the sensor elements.
According to a third general aspect, the present invention is based on the general technical teaching of providing a method of investigating a sample for sensing at least one probe molecule, wherein the above sensor device according to the invention is subjected to the sample and the local deflections at each of the sensor elements of the sensor device are read-out with an optical profilometer device. The measurement of local deflections preferably comprises an image processing of curvature patterns, e.g. light interference patterns measured on one of the surfaces of the sensor element.
Further details and advantages of the invention are described in the following with reference to the attached drawings. The drawings show in:
FIGS. 1A and 1B: schematic cross-sectional views of a first and a second embodiment of a sensor device according to the invention;
FIGS. 7A and 7B: schematic illustrations of supporting structures used according to the invention;
The preferred embodiments of the invention are described in the following with reference in particular to the design of the sensor element of a sensor device, the structure of the sensor device with a sample chamber and the structure and operation of the measuring device according to the invention. Details of manufacturing and operating the sensor devices are not described as far as they are known from conventional micro-mechanical systems, like e.g. micro-mechanical sensor devices. It is emphasized that the attached drawings represent schematic, enlarged illustrations. Details as well as relative and absolute sizes of the sensor and measuring devices can be adapted to the particular application of the sensor device by the skilled person.
The holding body 10 and the sensor element 20 are made e.g. from Si, SiN or plastics (e.g. plastic “SU 8”) as it is known from prior art cantilever-based sensor devices. The components 10 and 20 are integrally fixed to each other. The sensor element 20 is formed as a strip like a conventional cantilever with a length of e.g. 750 μm to 1 mm and a width in the range of 50 μm to 120 μm. The thickness of the sensor element 20 is typically selected in the range of 500 nm to 5 μm. The integral structure of the holding body 10 with the sensor element 20 is formed e.g. with reactive ion etching or wet chemical etching.
The sensor element 20 has an upper surface corresponding to one surface of the holding body and an opposite lower surface. Typically the sensitive layers 31 are arranged on the lower surface along the length of the sensor element 20, while the upper surface is free for optical measurements. Alternatively, the sensitive layers 31 are arranged on both surfaces along the length of the sensor element 20 as shown in
The thickness of the sensitive layers is selected in the range of 0.1 nm to several μm, e.g. 10 μm. The materials, number, sizes and positions of the sensitive layers 31 are selected in dependence on the particular application of the sensor device 100. The sensitive layers comprise a material, which changes properties, in particular the surface energy of the sensor element 20 in response to a physical or chemical interaction with a probe substance to be sensed (functionalization of the surface). Changing properties may comprise e.g. surface properties, swelling, conformation changes, electrostatic interactions etc. As an example, for sensing water vapour, the sensitive layer 31 consist of the polymer PPAA (plasma polymerized polyallylamine), which is expanding in response to an adsorption of water. Further materials for the sensitive layer comprise e.g. self-assembled monolayers, DNA-RNA- or PNA molecules, proteins, antibodies, aptamers etc.
The strip-shaped sensor elements 20.1, 20.2 . . . of
According to
The sensitive layers 31 are deposited on the sensor elements 20.1, 20.2, . . . , e.g. with a jet printing technique like the ink jet technique. With jet printing, sensitive layers 31 with a lateral size of about 50 μm can be deposited. With empty, non-functionalised portions having the same dimension between the sensitive layers 31, about ten to fifteen sensitive layers 31 could be arranged along the length of the sensor element 20.1 from the holding body 10 to the free apex thereof.
The sensitive layers 31 are deposited on the sensor element 20 according to the predetermined positions of the through-holes 13, while the reference portions 22 of the sensor element 20 remain empty for reference purposes. With a straight double-row of through-holes 13, the top-view of
The embodiment of
Typically, the web profiles 23 have a thickness in the range of 100 nm to 5 μm and a width in the range of 500 nm to 20 μm. The web profiles 23 are integrally formed with the sensor element 20. Advantageously, the web profiles 23 could be used as calibration structures described above.
According to the embodiment of
Furthermore, this embodiment can be adapted for one-way use. The sample could be introduced into the sample chamber 50, while the outer surface remains unexposed. The complete sensor device 100 could be adjusted in a measurement device for read-out of the local sensor element deflections.
The interferometer device 300 comprises a light source 310, a beam splitting element 320, a shiftable mirror 330, an imaging optic 340, a camera device 350 and a control device 360. The interferometer device 300 is e.g. a white light interferometer or an interferometer with one or more single wave-lengths, like e.g. a phase shift interferometer. Preferably, the light source device 210 comprises a laser source emitting at one or multiple wavelengths. The laser source may comprise one single laser emitting at a plurality of wavelengths or a plurality of lasers with different wavelengths. The camera device 350 comprises a CCD camera with a CCD matrix including at least one row of 300 pixels.
If the sensor device 100 is equipped with the deflection device 40 (dashed lines), a feedback loop 370 can be provided wherein the deflection device 40 is controlled with the control device 360 for a local compensation of deflections at the sensor elements 20 until all surfaces of the sensor elements 20 are plane. This condition can be monitored by imaging the interference pattern of the surfaces of the sensor elements 20 with the camera device 350.
Investigating a sample for sensing at least one probe molecule in the sample comprises the following steps. With a preparation step, a first reference image is taken including the interference pattern of the non-loaded sensor elements 20. Ideally, the interference pattern does not show any interference stripes, as all surfaces of the sensor elements 20 are plane and perpendicular to the beam path of the illumination light in the interferometer device 300. After storing the reference image, the sample to be investigated is introduced into the sample chamber 50 via the sample inlet 52. In response to an interaction of the probe substance to be sensed with at least one of the sensitive layers on the sensor elements 20, a local deflection is caused. Accordingly, the interference pattern is changed.
Finally, the calculated local curvature radii are compared with reference values stored e.g. in the control device 360. As a result of the comparison, the positions of sensitive layers showing an interaction are provided leading to a listing of probe substances detected in the sample.
The confocal microscope 400 is used like a conventional scanning profilometer. It comprises a light source 410, a beam splitting element 420, a shiftable objective lens 430, apertures 440 for confocal imaging, a camera device 450 and a control device 460. Preferably, the light source 410 comprises a laser source emitting at one or multiple wave-lengths. The objective lens can be adapted for providing multiple illumination paths towards the sensor elements allowing a parallel reading of the sensor elements. The camera device 450 comprises a e.g. CCD chip.
Evaluating a curvature pattern of the sensor elements with the confocal microscope 400 is based on the following concept. The sensor element is illuminated with illumination light from the light source 410. With the first aperture and the objective lens 430, at least one light spot is formed on the at least one sensor element. Depending on a local bending state of the sensor element, i.e. depending on the relative position of the surface of the sensor element and the focus of the illumination light, the light spot has a certain size or intensity. With various positions of the shiftable objective lens 430, a plurality of light spot sizes or intensities (e.g. 100 to 200) can be measured with the camera device 450. With a complete confocal imaging, a stack of sectional images is obtained. Accordingly, local focus levels of the sensor element can be calculated. On the basis of all focus levels of all sensitive layers of all sensor elements, a complete topography or image can be calculated and evaluated in analogy to the above interferometric measurement.
The features of the invention disclosed in the above description, the drawings and the claims can be of significance both individually as well as in combination for the realization of the invention it its various embodiments.
| Number | Date | Country | Kind |
|---|---|---|---|
| EP 06007850.8 | Apr 2006 | EP | regional |
