EP2140298A2

EP2140298A2 - Concept for detecting images with sub-wavelength resolution

Info

Publication number: EP2140298A2
Application number: EP08735058A
Authority: EP
Inventors: Wladimir Tschekalinskij; Stephan Junger; Norbert Weber
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2007-04-05
Filing date: 2008-04-07
Publication date: 2010-01-06
Also published as: WO2008122430A9; DE102007016588A1; JP2010524020A; US20100072353A1; US8300307B2; DE102007016588B4; JP5108936B2; WO2008122430A2

Description

Concept for capturing images with subwavelength resolution

description

The present invention relates to image sensors for capturing sub-wavelength resolution images and methods of making the same.

Already a few decades ago, the Russian researcher V. Veselago predicted an existence of materials with negative refractive indices n = co / c = Vε _r μ _r , where Co means the speed of light in a vacuum and c the speed of light in a material. Furthermore, ε _r denotes the dielectric conductivity or permittivity and μ _r the magnetic conductivity or permeability of the material. With a negative refractive index n, the Poynting

Vector S in the opposite direction of the wave vector k and the wave vector, the electric field strength E and the magnetic field strength H form a left-handed tripod. Therefore, materials with a negative refractive index are also referred to as so-called left-handed materials.

The opposite directions of the Poynting vector S and the wave vector k result in energy transport against wave propagation. In the transition from a left-handed material to a medium with positive refractive index n, the light is refracted not only towards the solder, but even beyond.

Such so-called left-handed materials can be obtained if one has both a negative permittivity ∈ _r and a negative permeability μ _r for a material, so that the refractive index n becomes negative. With such a left-handed material, for example, an ideal lens or a so-called super-lens can be built. This is characterized in that a point light source has a punctiform image, so works completely diffraction-free.

A classical optical system, as shown in Fig. 12, is always diffraction limited.

FIG. 12 shows an objective 10. A plane with an object to be imaged, ie an object plane 12, is located at a working distance or object-side distance fi to the objective 10. An image plane 14 is located at an image-side distance or image distance f ₂ to the objective 10. If an object or the object plane 12 is located in an object space with the refractive index n, then the arrangement shown in FIG. 12 results in a maximum resolution between two points at a distance d of

^ 0.6U d ^≥ : (1)

where λ is the wavelength of the light illuminating the object plane 12 and n'sinα is the numerical aperture of the objective 10.

It can be seen from equation (1) that there are two classic ways to increase the resolution of the optical system shown in FIG. On the one hand, it is possible to choose an exposure with a light of shorter wavelength λ, as in lithography with UV light or X-ray light, or it is possible to use so-called immersion liquids with an increased refractive index n relative to air in the object space, ie in the space to the left of the objective 10. However, even with high refractive index immersion fluids n, this is less than two, and thus the generally achievable resolution is in the range of about one-half wavelength λ / 2. Optical systems with a resolution in the sub-wavelength range, ie with resolutions which are significantly smaller than λ / 2, have already been demonstrated in the past. The diffraction limit described above can be circumvented, for example, with methods of near-field optics. Common near-field microscopes have a resolution of less than λ / 10.

At an observation distance to the objects, which is much smaller than the wavelength λ of the light, classical diffraction theory is no longer valid. Thus, the resolution can not be limited due to diffraction here. That is, near-field microscopes virtually bypass the Rayleigh criterion and thus obtain resolutions below λ / 2. This area of the light field that is close to an object to be examined is called the near field. In order to gain additional information from the near field, it is necessary to convert so-called evanescent field components into propagating field components. An evanescent field is generally the non-propagating component of the near field. The evanescent field exponentially decays to the surface normal of the radiating body. Each illuminated object thus generates an evanescent and a propagating field. A purely evanescent field can be observed, for example, in the case of total reflection. If an incident light beam is totally reflected at an interface of an optically denser medium to an optically thinner medium, the field can not abruptly become zero due to the condition of continuity on the side of the optically thinner medium, but falls exponentially into the half space of the optically thinner medium from. In general, the evanescent field has already disappeared at a distance of about λ / 2 from the interface of the two optical media. This field in particular contains information about structures below the classical resolution limit. In order to convert evanescent field components into propagating field components, one can, for example, introduce a scattering center into the near field. In this scattering center, the evanescent field excites dipole oscillations, so that again evanescent and propagating field components arise as a result of the interaction of the scattering center with the near field of the object. Another possibility is the scanning of an object surface with an optical probe with a single-mode fiber, at the end of which there is a diaphragm with a hole diameter of about 40 nanometers. The light emerging from this waveguide strikes the object plane, thereby changing its evanescent field. A remote receiver and its signal processing registers this change in the evanescent field, from which refractive index n and transmission and reflection coefficients can be calculated. Various methods for measuring the near field are described, for example, in the thesis "A High-Resolution Near-Field Optical Probe for Fluorescence Measurements on Biological Samples" by Hein- rich Gotthard Frey.

Only in recent years has the theoretical properties of the initially described left-handed materials been proven in experiments. Optical left-handed structures have been realized that map object structures in the sub-wavelength range, so-called superlenses. These optical structures with negative refractive index transmit the evanescent field of an exposed object which carries information about higher spatial frequencies (eg of structures which are smaller than the wavelength λ of the light used), and make it almost loss-free from the object plane into the image plane from. These left-handed materials reconstruct the evanescent fields of subwavelength structures.

At present, there are no known materials of negative refractive index or negative permeability μ _r in nature. Artificially, these properties can be however, with so-called metamaterials or photonic crystals having small periodic structures that are significantly smaller than the exposure wavelength λ, such that the electromagnetic waves only sense effective material properties. Some experiments confirming the theory of left-handed materials are described in the following publications. Hyesog Lee, Yi Xiong, Nicholas Fang, Werayut Srituravanich, Stephane Durant, Muralidhar Ambati, Cheng Sun and Xiang Zhan: "Realization of optical superlens imaging under the diffraction limit", Wenshan Cai, Dentcho A. Genov, and Vladimir M. Shalaev: "Superlens based on metal-dielectric composites", Gnnady Shvets: "Band engineering using electrostatic resonances applications to super-lensing".

Magnified images with super lenses are also known, the publication "Magnifying Superlens in the visible frequency range" e.g. shows theoretical and experimental results.

While the use of left-handed materials to achieve sub-wavelength resolution is known, suitable image sensors are currently lacking to capture complete two-dimensional images of sub-wavelength specimens. In the near-field microscopes described above, an object sample must be scanned point by point, so that a point-by-point two-dimensional scanning of the sample surface is required to obtain a complete image. In addition, the transfer of the evanescent field to a photoreceiver with the help of additional optics, such as waveguides or lenses is relatively problematic. The weak and with increasing distance exponentially decaying evanescent field is further attenuated by the transfer to the photoreceiver, resulting in a low accuracy of measurement. In addition, in a conventional Nahfelddedektion only static objects can be detected, since either an exposure a film-like layer or a scanning of the object is required.

The object of the present invention is thus to provide an improved concept for the detection of an object in the sub-wavelength range, which can provide a complete image of the object without a point-by-point scanning of the object, or a concept for increasing a resolution of diffraction-limited To provide optics.

This object is achieved by a microscope with an integrated image sensor having the features of patent claim 1, a method according to claim 13 and an optical device according to claim 18.

The finding of the present invention is that an optical resolution improved with respect to diffraction-limited optics can be achieved by imaging an object by means of an optical structure with a negative refractive index onto a pixel array which is located in an image plane of the optical structure. For this purpose, according to embodiments, an object to be observed is brought in an object plane in a near-field distance to a first side of the optical structure with negative refractive index, wherein the object to be viewed is illuminated with monochromatic light. The near-field distance between the object and the first side of the optical structure is smaller than the wavelength λ of the monochromatic light according to embodiments. The evanescent field resulting from the illumination of the object at the object is transmitted from the first side of the optical structure to a second opposite side thereof. At a near-field distance smaller than the wavelength of the monochromatic light, the pixel array is located along the second side of the left-handed optical structure to detect the transmitted evanescent field and then process it further. According to embodiments, the image sensor having the negative refractive index optical structure and pixel array is fabricated in a CMOS process. In this case, the pixel array has a two-dimensional array of PN junction sensors, in particular photodiodes. The individual pixel elements of the pixel array are spaced to detect the evanescent field in the sub-wavelength range.

At a distance smaller than the wavelength, e.g. less than 1.2 μm, preferably less than 1 μm, and more preferably less than 0.8 μm, the second side of the optical structure extends from the pixel array. According to exemplary embodiments, the optical structure has a combination of structured metal layers and dielectric layers, so that the structuring achieves a negative refractive index for the wavelength of the monochromatic light or the exposure wavelength. The combination of the patterned metal layers and the dielectric layers may be formed such that the optical left-handed structure is a metamaterial or a photonic crystal.

In the following, it is assumed that a metamaterial is formed from a layer stack of different dielectric materials, wherein a metal layer with microapertures is placed on an uppermost dielectric layer. In this case, a microaperture is to be understood as meaning a structured opening with dimensions smaller than the illumination wavelength.

In the following, a structure which substantially has the characteristics of a photonic crystal will be referred to as a photonic crystal. In particular, structures are meant to have periodic (metallic) structures with dimensions smaller than the exposure wavelength in a dielectric material. They therefore show only like optical properties, such as room-angle selectivity and spectral selectivity.

The optical structure preferably comprises at least one lowermost CMOS metal layer (CMOS metal layer) closest to the pixel array, in which the microapertures are structured, and / or a metal layer in which, in addition to the microapertures, metallic, electrical connections or metal layers are formed ., Tracks between circuit elements (eg transistors) of the integrated image sensor are structured. A thickness of a CMOS metal layer is in embodiments in a range greater than or equal to 90 nm.

An advantage of an image sensor according to embodiments is that it enables resolution and detection of structures much smaller than the exposure wavelength λ. An image sensor according to embodiments can be used, for example, to simultaneously detect the entire image of an object to be examined in a microscope with sub-wave resolutions and to convert it into an electrical video signal. A microscope with sub-wave resolutions according to embodiments does not need a lens, since an imaging optics is already integrated in an image sensor according to embodiments. It is possible to amplify, process and digitize an evanescent field converted into an electrical signal directly in the sensor.

Another advantage of embodiments of the present invention is that resolution of a diffraction limited system can be increased. This can be done by mounting an image sensor according to embodiments in an image plane of the diffraction-limited optics. As a result, structures smaller than about half a wavelength can be resolved. If an image sensor according to embodiments of the present invention is used in conjunction with a camera with diffraction As a result of the pronounced angular selectivity of the optical left-handed structures, which leads to a separation or suppression of interfering diffraction components, resolution in the sub-wavelength range can be achieved.

Preferred embodiments of the present invention will be explained in more detail below with reference to the accompanying drawings. Show it:

1 shows a schematic flow diagram of a method for generating an image of an object according to an embodiment of the present invention düng;

FIG. 2 is a perspective view of an image sensor according to an embodiment of the present invention; FIG.

3 is a perspective view of an image sensor according to another embodiment of the present invention;

FIGS. 4a-c are side views of a negative refractive index meta material showing an object plane, an image plane of the metamaterial, and an evanescent field trace, according to embodiments of the present invention for different evanescent field generations;

5 shows a side view of an image sensor with metamaterial according to an embodiment of the present invention;

6a is a side view of a negative refractive index photonic crystal, and an object plane and an image plane of the photonic crystal, according to an embodiment of the present invention. FIGS. 6b and 6c show modifications of FIG. 6a according to FIGS. 4b and 4c;

7 is a plan view of a metal layer with split-ring resonators according to an embodiment of the present invention;

8 is a side view of a photonic crystal image sensor according to an embodiment of the present invention;

9 shows a side view of a layer stack of pixel array, metal layers and dielectric layers produced using CMOS technology, according to one exemplary embodiment of the present invention;

10 is a schematic representation of a microscope with an image sensor according to an exemplary embodiment of the present invention;

11 a shows a schematic representation of an optical device having a diffraction-limited objective and an image sensor according to an exemplary embodiment of the present invention;

Fig. IIb is a schematic representation of a detail of the illustration of Fig. IIa to illustrate the angular selectivity properties of the photonic crystal in Fig. IIa; and

Fig. 12 is a schematic representation of a conventional diffraction-limited optics.

With regard to the following description, it should be noted that in the different embodiments identical or functionally identical functional elements have the same reference numerals and thus the descriptions of these functional elements in the various embodiments shown below are interchangeable.

1 shows a schematic flow diagram of a method for producing an image of an illuminated or luminous object in the sub-wavelength range of the illuminating light.

In a first step Sl, the object to be examined or the object to be examined, which is located on a slide, is illuminated. In this case, the illumination of the object takes place according to exemplary embodiments with a monochromatic light having a wavelength λ of, for example, less than 1.2 μm.

In a second step S2, a first side of an optical structure with negative refractive index n is arranged at a near field distance di to the object.

The two steps S1 and S2 can be matched to one another in various ways, so that an evanescent light field caused by the object's illumination on the first side of the optical structure in a subsequent step to be discussed, the opposite second side of the optical structure can reach. Three possibilities will be clarified in more detail below with reference to FIGS. 4a to 4c, for which reason they will be discussed very briefly at this point.

A first possibility consists, for example, in arranging an interface from an optically denser to an optically thinner medium, such as a prism surface, near the front of the optical structure and illuminating the prism surface from the optically dense surface. rem medium is made at an angle to the prism surface at which total reflection occurs at the prism interface. Under these conditions, an evanescent field is formed in the optical thinner medium between the prism boundary surface and the front surface of the optical structure, illuminating an object to be examined between the prism interface and the front surface. In order for the evanescent field modulated by the object in phase or amplitude to reach the first side of the negative refractive index optical structure n, a distance di between the object and the first side of the optical structure may be used, which is preferably smaller than the wavelength of the light used, for example, monochromatic light, such as a distance less than 1.2 microns.

A second possibility, for example, is that the object is in optical contact with the front of the optical structure, e.g. the same touches, and is illuminated by the side facing away from the optical structure. In this case, an evanescent field of sub-wavelength openings is created at the front of the optical structure. The amplitude and phase of the evanescent field arising at a respective one of the subwavelength apertures depend on electrical and / or magnetic permeabilities of the object having the latter at the local or lateral location which is in direct optical contact with the respective aperture.

A third possibility is, for example, that the object is arranged in a near field to the front of the optical structure, while it is illuminated from the side facing away from the optical structure, for example obliquely, in which case, for example, an evanescent field is formed on a side of the object facing the optical structure, which then reaches the subwavelength apertures on the front side of the optical structure and originates from the same from the far field. is filtered out. In order for the evanescent field of the object to reach the first side of the optical structure with negative refractive index n, a distance between the object and the first side of the optical structure may preferably be smaller than the wavelength of the light used for illumination, for example monochromatic light, used, such as a distance less than 1.2 microns.

In a third step S3, the evanescent field of the object, which has reached the first side of the optical structure or arises there, is transmitted from the first side to a second side of the optical structure. Transmission of the evanescent field is possible through the use of a negative refractive index optical material in a wavelength range of interest. Thus, the evanescent field can propagate from the first side to the second side of the optical structure.

In a fourth step S4, the evanescent field on the second side of the optical structure is detected by a pixel array. In this case, the pixel array along the second side of the optical structure is arranged at a distance d ₂ , which is preferably smaller than the wavelength of the illuminating light, ie d ₂ <λ. As mentioned at the beginning, an evanescent field of an object sounds relatively fast, hence its name. This is the reason that both the first distance di and the second distance d _{2 should} each be selected smaller than the exposure wavelength λ for a near-field detection.

Thus, FIG. 1 describes a method for producing an image of an object, comprising a step of exposing the object to a first side of a negative refractive index optical structure at a near field distance to the object such that an evanescent field from the object is the first side and the evanescent field is transmitted from the first side to a second side of the optical structure opposite the first side, and a step of detecting the evanescent field on the second side by a pixel array.

FIG. 2 shows an image sensor 20 according to an exemplary embodiment of the present invention for carrying out the method described with reference to FIG. 1.

The image sensor 20 has an optical structure 22 with a first side 22a and a second side 22b opposite the first side. The optical structure 22 has a negative refractive index n, ie n <0. Furthermore, the image sensor 20 comprises a pixel array 24 that extends at a predetermined distance d ₂ along the second side 22 b.

The optical structure 22, which may also be referred to as a left-handed structure, is _{configured in} accordance with embodiments to transmit an evanescent light field E _EVan from the first side 22a to the second side 22b of the optical structure 22. The pixel array 24 is designed to capture a two-dimensional image and has, according to embodiments (X'Y), sensor elements 26 in order to be able to display a two-dimensional image with X'Y pixels. The sensor elements 26 are designed according to embodiments as PN junction sensors. In this case, a PN junction sensor means a sensor with a PN junction realized by different doping. According to embodiments, the PN junction sensors 26 are formed as photodiodes. That is, the pixel array 24 is a photodiode array according to embodiments.

Adjacent sensor elements 26 of the pixel array 24 are each arranged at a distance smaller than the exposure wavelength λ according to embodiments 24 in order to be able to acquire images of an object to be examined in the sub-wavelength range. However, it may also be that the optical structure has magnifying properties that it to increase the pixel size, such as due to the higher index of refraction of the material between the optical structure and the sensitive area of the pixel sensor, with reference to the article "Magnifying Superlenses in the Flexible Frequency Ranks" by Smolyaninov, Hung and Davis in Science, 315 , March 23, 2007, pp. 1699-1701.

The predetermined distance d _{2 of} the pixel array from the second side 22 b of the optical structure 22 is set such that an evanescent field transmitted from the first side 22 a to the second side 22 b and exiting from the second side 22 b from the pixel array 24 can be detected. For example, exposure wavelengths smaller than 1.2 μm are used, for example together with CMOS pixel sensors. In this case, the distance d ₂ would also be selected, for example, smaller than 1.2 microns. In other embodiments, the distance d _{2 is} less than 1 μm or even less than 0.8 μm.

Furthermore, a negative refractive index optical structure, that is, a left-handed material can be constructed by means of a so-called metamaterial. A metamaterial is a composite material whose electromagnetic material properties depend on its structure, rather than the specific properties of the material or materials of which it is made. In the following, it is assumed that a metamaterial is formed from a layer stack having a sandwich structure of a plurality of dielectric layers, which have suitable dielectric constants and layer thicknesses, wherein a metal layer with microapertures is placed on an uppermost dielectric layer. Micro-aperture should be understood to mean a structured aperture with dimensions smaller than the exposure wavelength. An image sensor 30 according to an embodiment having a metamaterial thus constructed is shown in FIG.

The image sensor 30 in FIG. 3 has an optical structure of a layer stack 31 at a distance d ₂ <λ from the pixel array 24. The layer stack comprises a first dielectric layer 32 which forms the second side of the optical structure 31, a second dielectric layer 34 and a third dielectric layer 36. Furthermore, the layer stack 31 on the third dielectric layer 36 comprises a metal layer 38 in which Microapertures 40 are located. The microapertures 40 are, for example, circular micro-openings. In general, a size or a diameter of the microapertures or of the micro-openings 40 and a spacing of two adjacent microapertures 40 relative to one another are in the sub-wavelength range. That is, both the diameter of the microapertures 40 and distances between two adjacent microapertures 40 are smaller than the exposure wavelength, ie smaller than, for example, 1.2 μm.

The metal of the metal layer 38 is, for example, metals used in CMOS processes, e.g. Aluminum or copper. Furthermore, it should be pointed out at this point that the microapertures shown in circular fashion in FIG. 3 can also be other structures such as rectangles, hexagons, grid lines or the like in the sub-wavelength range.

According to exemplary embodiments, the sandwich structure comprising the first dielectric layer 32, the second dielectric layer 34 and the third dielectric layer 36 is an SiO ₂ / SiC / SiO ₂ structure on which the metal layer 38 is located. That is, both the first dielectric layer 32 and the third dielectric layer 36 are SiO ₂ layers (SiO ₂ = silicon dioxide). The dielectric layer 34 is a SiC layer (SiC = silicon carbide). The permittivity ε _r , _S ic of the SiC layer 34 is approximately equal to the negative permittivities ε _r , _S io 2 of the SiO 2 layers 32, 36, ie ε _{r / S} ic = -ε _r , _S i02-

Other dielectric layers 32, 34, 36 with suitable material properties, which lead to a negative refractive index, are of course also conceivable.

A side view of the metamaterial or the layer stack 31 from the dielectric layers 32, 34, 36 and the metal layer 38 is shown in FIG. 4a.

In applications of an image sensor according to embodiments, the first side 22a of the optical structure or of the metamaterial 31 formed by the metal layer 38 is located in a near field distance di from an object plane 41, so that an evanescent field 42 emanating from the object to be examined is the first side 22a or reaches the metal layer 38. Due to the special properties of the optical structure 31 (negative refractive index), the evanescent field 42 is amplified during the transmission from the first side to the second side 22b of the optical structure 31, as is indicated schematically in FIG. 4a. When exiting at the second side 22b of the optical structure 31 or at the interface of the dielectric layer 32, the transmitted evanescent field 42 again decreases rapidly. Therefore, the plane or image plane 44 where the evanescent field is detected should be no further than d ₂ <λ from the second side 22 b of the optical structure 31.

As shown in FIG. 4 a, the field strength of the evanescent field 42 exponentially drops between the object plane 41 and the metal layer 38, and then strikes the optical structure or superlens 31, which consists of the structured metal layer 38 and the dielectric layer Layers 32, 34, 36 consists. This optical structure 31 transmits the evanescent field 42 without losses to the second side 22b, where the field strength of the transmitted evanescent field 42 dies down again. Therefore, the distance d ₂ between the second side 22b and the image plane 44 where the evanescent field is registered should be smaller than the exposure wavelength λ, so that the field strength is not so small.

4a thus relates to the above-mentioned third possibility for evanescent field generation, according to which the object 41 is arranged at a near field distance di to the front side 38 of the optical structure 31, while it is illuminated by the side facing away from the optical structure 31; namely, for example, obliquely to the front side 38, in which case, for example, an evanescent field 42 is formed on an optical structure facing side 41 of the object, which then reaches the sub-wavelength openings on the front side 38 of the optical structure 31 and from the super lens openings in the same from the Far field is filtered out.

Fig. 4b refers to the aforementioned first possibility for evanescent field generation, according to which an interface 45 of an optically denser medium, namely here a prism 46 to an optical thinner medium 47 near the front side 22a of the optical structure 31 is arranged and an illumination of the prism surface 45 is made of the optically denser medium at an angle α to the prism surface 45, in which total reflection at the prism boundary surface 45 is formed. Under these conditions, in the optical thinner medium 47 between the prism boundary surface 45 and the front side 22a of the optical structure 31, the evanescent field 42 is formed, illuminating an object 48 to be examined between the prism boundary surface 45 and the front side 22a. The evanescent field 42 modulated by the object 48 in phase or amplitude reaches the first side 22a of the optical structure 31 with negative refractive index n. For example, Fig. 4c refers to the aforementioned second possibility for evanescence field generation. The object 48 is here in optical contact with the front side 38 of the optical structure 31, such as by touch. Illuminated is from the optical structure 31 side facing away. In this case, the evanescent field is formed at the subwavelength apertures on the front face 22a of the optical structure 31. The amplitude and phase of the evanescent field formed at each of the subwavelength apertures depend on electrical and / or magnetic permeabilities of the object 48 having the latter at the local or lateral position which is in optical contact with the respective opening 49.

FIG. 5 shows a side view of the integrated image sensor 30 already shown in FIG. 3. The chip or image sensor 30 comprises a structured metal layer 38 and three dielectric layers 32, 34, 36 which together form a material having a negative dielectric constant ε _r . In FIG. 5, the object plane 41 is very close, ie at a near field distance, to the metal layer 38, the imaging plane 44 in turn lies very close to the photodiode array 24 or the photodiode array 24 lies directly in the imaging plane 44. The photodiode array 24 extends at a distance d ₂ <λ along the second 22 b side of the optical structure 31.

In addition to the described metamaterials, another way of producing a left handed negative refractive index material is to produce a three-dimensional periodic structure whose periodically arranged elements have dimensions and spacings which are smaller than the wavelength λ of the illuminating light. Such three-dimensional periodic structures may be, for example, what are known as photonic crystals or optical structures which are present in accordance with exemplary embodiments how to get a photonic crystal. Hereinafter, such structures are generally referred to as photonic crystals.

Photonic crystals include structured semiconductors, glasses or polymers and, by virtue of their specific structure, force light to propagate in the medium in the manner necessary for a device function. They are periodic dielectric and / or metallic structures whose period length is adjusted to affect the propagation of electromagnetic waves in a similar manner as the periodic potential in semiconductor crystals affects the propagation of electrons.

An optical structure 52 that behaves virtually like a photonic crystal is shown in FIG. 6a.

The negative refractive index optical structure 52 shown in FIG. 6a includes periodically arranged metal layers 60 in a dielectric medium 62, such as SiO ₂ . The metal layers 60 have, for example, micro-openings, as indicated in the side view of the optical structure 52 in FIG. 6a. The geometric shape of these micro-openings can be varied and depends on the desired electromagnetic properties of the optical structure 52. Possible, for example, circular micro-openings whose dimensions and distances from one another are on the order of magnitude of the exposure wavelength λ or smaller than the exposure wave λ. A distance 1 between adjacent metal layers 60 is also in the order of magnitude of the exposure wavelength λ or smaller than this.

Here again, in order to transport an evanescent field originating, for example, from the object plane 41 to the first side or originating on the first side, from the first side 22a of the optical structure 52 to the second side 22b of the optical structure 52 first side 22a at a near field distance di smaller than the exposure wavelength λ from the object plane 41. The image plane 44, in which preferably the pixel array 24 is placed, also extends, in the exemplary embodiment shown in FIG. 3 a, the optical structure 52 at a distance 0 ₂ smaller than the exposure wave λ from the second side 22 b.

The field intensity profile of the evanescent field results in the embodiment shown in FIG. 6a in a similar manner as already described above with reference to FIG. 4a. That is, by the optical structure or the photonic crystal 52 having a negative refractive index, an evanescent field arriving at the first side 22a is amplified upon transmission through the optical structure 22 toward the second side 22b thereof. This effect of amplifying near-field waves is described, for example, in C. Lou et. al, "Subwavelength imaging in photonic chrystals".

Similar to FIGS. 4a-4c, FIGS. 6b and 6c show further evanescent field generation possibilities in the context of a photonic crystal. For further details, reference is made to the description of Figures 4b and 4c.

FIG. 7 shows a possible embodiment of microelements realized in the metal layers 60.

FIG. 7 can be seen as a top view of the negative refractive index optical structure 52. In a dielectric medium 62 (shown in black) microelements 70 (shown in white) are introduced into metallic layers. FIG. 7 shows by way of example a section with six periodically arranged microelements 70, which have a so-called split-ring-resonator structure. 7 thus shows a plan view of a possible realization form of the various layers 60 with metallic microelements 70 shown in a side view in FIG. 6 a. The dimensions of the micro-element 70 are significantly smaller than the exposure wavelength λ. Changing the ratio of the radii of the outer and inner circle of a split-ring resonator 70 changes the corresponding resonator wavelength. It should be emphasized that other structures, such as so-called LC-Loaded Transmission Lines, are possible, wherein it is important that an overall transmission is as high as possible.

FIG. 8 shows in summary a built-in image sensor 80 realized using CMOS technology, using an optical structure 52 described with reference to FIGS. 6 and 7, which will not be described in more detail here, since it is only structurally configured. used to distinguish left-handed material 52 from the image sensor described with reference to FIG. 5.

Image sensors based on the previously described left-handed materials (metamaterials and three-dimensional photonic crystals) can be realized with CMOS processes, such as a CMOS opto-process, without the need for additional process steps or further processing.

A method of fabricating an integrated image sensor on a substrate comprises, according to embodiments, a step of generating a photodiode array 24 on a substrate surface of the substrate and applying a negative refractive index optical structure to the photodiode array such that the photodiode array is at a predefined distance d ₂ extends along the optical structure, wherein the generating and the applying are parts of a CMOS process.

According to exemplary embodiments, the application of the optical structure comprises applying a layer stack comprising at least one dielectric layer and one metal layer, wherein the at least one metal layer comprises microstructures. having structures that have dimensions and spacings between two adjacent microstructures that allow an evanescent field to be transmitted through the optical structure.

An intermediate product of a CMOS manufacturing process of an image sensor according to embodiments is shown schematically in FIG. 9.

The integrated image sensor, not yet produced, shown in FIG. 9 comprises a substrate 90, in particular a semiconductor substrate, in which a photodiode array 24 is introduced, wherein FIG. 9 merely shows a photodiode 26 of the photodiode array 24 by way of example. In this case, the photodiodes 26 are arranged in a plane 92 which corresponds to the image plane 44, or at least runs very close to the image plane 44.

The unfinished optical structure in FIG. 9 has a layer stack of metallic layers 94 and dielectric layers 96. FIG. 9 merely shows by way of example four metallic layers 94-1 to 94-4 and three dielectric layers 96-1 to 96-3. Depending on the embodiment, the number of layers may differ from the example shown in FIG. 9.

In the case of a metamaterial with a metal layer on the first side 22a and a subsequent layer stack of three dielectric layers, therefore, only the metal layer 94-4 will remain and the unneeded metal layers 94-1 to 94-3 will become part of the CMOS process completely removed. In addition, the upper metal layer 94-4 is still provided with the described micro-openings. In conjunction with the three remaining dielectric layers 96-1 to 96-3, which have corresponding dielectric constants ε _r , a left-handed material, in particular a metamaterial, is produced with negative ver refractive index, as has already been described above with reference to FIGS. 3 to 5.

On the other hand, in current CMOS processes, it is also possible to structure the underlying metal layers 94-1 to 94-3 so finely that resulting microstructures or microelements are arranged periodically and smaller than the exposure wavelength λ. This makes it possible to produce three-dimensional periodic structures with properties of photonic crystals directly on a chip. As already described above, the individual microelements or microstructures of the structured metal layers are smaller than the exposure wavelength λ, so that a three-dimensional photonic crystal is produced which acts as a left-handed material.

In all embodiments described so far the photodiode array 24 is preferably placed very close to the final layer of the structure 94-1, wherein the distance d ₂ is less than the exposure wavelength λ. Each individual photodiode 26 then registers only a corresponding proportion of the evanescent field of the object to be examined and, in conjunction with signal processing, as in conventional imaging sensors, a two-dimensional image of the object is produced (not shown in FIG. 9).

Schematic views of image sensors realized by CMOS processes with left-handed optical structures and pixel arrays according to exemplary embodiments have already been explained with reference to FIGS. 3, 5 and 8.

Having been explained in detail in the foregoing production and construction of image sensors according to embodiments of the present invention, applications of image sensors according to embodiments will be explained below with reference to FIGS. 10 and 11. FIG. 10 shows a microscope 100, which is in particular a sub-wavelength microscope or near-field microscope, according to an exemplary embodiment of the present invention.

Monochromatic light 104 emanating from a monochromatic light source 102 passes through an optical system 106, e.g. a condenser, and then illuminates a slide 108 having a structure to be examined or object 110 to be examined. At a near field distance di smaller than the wavelength λ of the monochromatic light 104, an integrated image sensor 20 according to embodiments is arranged. Alternatively, the sample 110 may also be in contact with a front side of the image sensor 20.

The image sensor 20 is coupled to an image processing system 112, which in turn is connected to an output device 114, such as a monitor. The left-handed optical structure 22, which may be a metamaterial or a photonic crystal, encompassed by the image sensor 20 transmits the evanescent field originating from the object 110 to be examined or dependent on the object 110 to be examined without circumcision of high spatial frequencies or spatial frequencies to the second side or rear side of the optical structure 22 or in the image plane. The photodiode array 24 is located at a distance d ₂ of less than a wavelength λ to the imaging plane 44 and to the second side 22b of the optical structure in order to minimize losses due to the rapidly decaying evanescent field. The image processing system 112 processes and digitizes the signal from each photodiode 26 sensed by the photodiode array and generates an image for the connected monitor 114.

Compared to conventional near-field microscopes, which scan objects only point by point with a very fine probe, For example, a sub-wavelength microscope with an image sensor according to embodiments of the present invention has the advantage that the entire image of the object to be examined can be detected and converted into an electrical video signal. Furthermore, moving objects such as bacteria cultures or the like can also be examined by means of an image sensor according to embodiments. When exposing a film-like layer or when scanning an object, motion capture is not possible.

Another application of an image sensor according to embodiments is to improve a resolution of diffraction-limited lenses. To this end, embodiments of the present invention provide an optical device having an objective, an optical structure having a negative refractive index, wherein a first side of the optical structure is arranged in an image plane of the diffraction-limited optics, and a pixel array extending along a predetermined distance one of the first side of the optical structure opposite second side extends. The lens is set, for example, to a distant object plane at a distance of, for example, more than five focal lengths.

Such an optical device 200 is shown schematically in Fig. IIa.

The optical device 200 comprises a diffraction-limited objective 202 with an image plane 204. The first side 22a of a left-handed optical structure 22 is arranged in the image plane 204 of the objective 202. At a predefined distance d ₂ from the second side 22b of the optical structure 22 is a pixel array 24, in particular a photodiode array. According to a preferred embodiment, the optical structure having a negative refractive index is a photonic crystal or a three-dimensional structured structure which looks like a photonic crystal as described above with reference to FIGS. 6-8.

Conventional optical systems have a limited resolution of about half a wavelength due to diffraction at the lens 202 and the diameter of the entrance pupil of the system, respectively. The Rayleigh criterion defines exactly the resolution limit for a lens, considering a black and white periodic line structure. The minimum distance d of two lines which can still be resolved is then defined such that the maximum of the diffraction image of a first slit coincides with a first minimum of the diffraction image of a second adjacent slit. This means for two points 206, 208 which are close together with respect to a viewing angle at infinity, such as e.g. two fixed stars, a light intensity distribution 210 in the image plane 204 of the lens 202 does not allow resolution of the two objects, since corresponding diffraction images or diffraction slices 212a, b overlap strongly. A diffraction image of each object point consists of several diffraction orders (k = 0, k = ± 1, k = ± 2, etc.) with minima and maxima, as shown schematically in FIG. IIa, so that corresponding diffraction slices 212a, b are formed.

With the aid of an image sensor 20 with a photonic crystal according to exemplary embodiments and a photodiode array 24, it is possible to reduce the diffraction components of the light emanating from the object points 206, 208 during the imaging by the photonic crystal onto the photodiode array, so that a light component higher diffraction orders (k = ± l, k = ± 2, etc.) is very low. Photonic crystals are highly selective with respect to the angle of incidence of light. The microelements of the photonic crystal 52 transmit incident light only in very narrow solid angle regions and strongly attenuate diffraction light originating from other directions. This can be explained as follows, with reference to Fig. IIb, which shows part of the mapping path of Fig. IIa, namely a part beyond the image plane, ie in the photonic crystal, and there only a laterally limited to a super lens aperture part. The openings 308 of the metal layers of the optical structure each have an opening diameter which generally corresponds to a maximum opening extent of an arbitrarily shaped opening 308. In the case of circular openings, the opening diameter thus corresponds to a circle diameter, with a rectangular opening of a rectangular diagonal, etc. According to exemplary embodiments, the openings 308 of adjacent metal layers 310 can be arranged laterally offset from one another by less than the opening diameter in order to achieve a solid angle selectivity of the optical structure. As in the case of FIG. 6, the openings 308 in the metal layers 310 of the photonic crystal are on a common axis 312, which may be parallel to the optical axis of the system of FIG. 11a, and also through a pixel, such as a Photodiode, 316 of the sensor 24 extends. The apertures 308 now together form a sub-wavelength waveguide 319 which achieves the aforementioned strong solid angle selectivity and which transmits the evanescent field resulting at the entrance-side aperture 308: to the pixel 316. Due to the sub-wavelength waveguide properties or the strong angle selectivity of the sub-wavelength waveguide 319, diffraction components of the transmitted light are suppressed so that no interfering diffraction light is incident on the sensor 24 or on the pixel 316. In this way, the sensor 24 "sees" through a type of "tube array" 52 with tube 319 the image of the diffraction limited objective 202 without the disturbing diffraction effects.

After the optical filtering by the photonic crystal 52, therefore, it is possible to still dissolve two adjacent light sources 206, 208 whose distance d is below the Rayleigh criterion, since the light field filtered by the photonic crystal 52 has a smaller fraction of diffraction light. as the light just behind the diffraction-limited lens 202. The photodiode array 24 registers the evanescent field, which includes less interfering diffraction light than the field directly behind the lens 202. Thus, of the pixel array 24, as indicated by reference 214, for example, only the 0th diffraction order of incident light are registered after higher diffraction orders have been filtered out by the optical crystal 52.

In summary, the present invention thus relates to image sensors which are constructed as an optical-electrical hybrid structure and comprise a left-handed material. This left-handed material may be formed, for example, by a metamaterial or a three-dimensional photonic crystal, both consisting of a sandwich-like form of microstructured metal layers and dielectric layers having defined dielectric constants ε _r and a photodiode array. Both image sensors with a metamaterial and image sensors with a photonic crystal can be integrated with CMOS technology without the need for additional process steps. An image sensor constructed in accordance with embodiments operates with a monochromatic light source having an exposure wavelength λ and may have a resolution of better than λ / 10.

The evanescent field, which is a weak field and whose intensity decreases exponentially with the distance from the illuminated object, can be detected immediately and without loss of the left-handed material by the photodiode array, since the distance between the imaging plane and the second side of the left-handed Material and the photodiode array according to embodiments is smaller than the exposure wavelength λ.

When using an image sensor according to embodiments together with a diffraction-limited lens in In the case of teleoscopic use, it is possible to achieve a resolution improvement due to the filtering of the evanescent field from the far field by the left-handed material, which is based on a separation or suppression of the interfering portion of the diffraction light.

In the field of (near field) microscopy, no conventional optics are needed to image an object. A combination of structured metal layers, dielectric layers, and a photodiode array enables a hybrid element that can resolve structures smaller than λ / 10. A point-by-point scanning of the object to be examined is not required.

Finally, it should be understood that the present invention is not limited to the particular components described or the illustrative procedures, as these components and methods may vary. The terms used herein are intended only to describe particular embodiments and are not intended to be limiting. When the singular or indefinite articles are used in the specification and claims, these also refer to the majority of these elements unless the context clearly makes otherwise clear. The same applies in the opposite direction.

Claims

claims

1. Microscope (100) with sub-wavelength resolution, having the following features:

a light source (102) for monochromatic light (104) having a predetermined wavelength (λ);

a slide (108) for an object to be examined (110); and

an image sensor (20; 30; 80) with

an optical structure (22; 31; 52) having a first side (22a) and a second side (22b) opposite the first side, the optical structure (22; 31; 52) having a negative refractive index (n); and

a pixel array (24) extending at a predetermined distance (d ₂ ) along the second side (22b) of the optical structure (22; 31; 52),

wherein the first side (22a) of the optical structure (22; 31; 52) of the image sensor (20) is located at a near field distance (di) smaller than the wavelength (λ) of the monochromatic light (104) to the slide (108).

The microscope of claim 1, wherein the optical structure (22; 31; 52) is adapted to form an evanescent field (42) from the first side (22a) to the second side (22b) of the optical structure (22; 31; 52).

3. A microscope according to claim 1 or 2, wherein the optical structure comprises a metamaterial (31).

4. A microscope according to claim 3, wherein the metamaterial (31) comprises a layer stack of dielectric layers

(32, 34, 36) and a metal layer (38) having micro-apertures (40), wherein dimensions of the micro-perturen (40) and distances between adjacent micro-apertures have orders of magnitude such that an evanescent field (42) at the first Side (22a) to the second side (22b) is transferable.

5. A microscope according to claim 4, wherein the dimensions and distances are smaller than 1.2 microns.

6. A microscope according to claim 1 or 2, wherein the optical structure comprises a photonic crystal (52).

7. A microscope according to claim 6, wherein the photonic crystal (52) is formed by a three-dimensional periodic structure comprising microelements (70), the spacings and dimensions of which have an order of magnitude such that an evanescent field on the first side (22a) second side (22b) can be transmitted.

8. A microscope according to claim 7, wherein the three-dimensional periodically arranged microelements are designed as split-ring resonators.

A microscope according to any one of the preceding claims, wherein the pixel array (24) comprises an array of PN junction sensors.

A microscope according to any one of the preceding claims, wherein the pixel array (24) is a photodiode array.

A microscope according to any one of the preceding claims, wherein the predetermined distance of the pixel array (24) from the second side (22b) is set such that one transmitted from the first side (22a) to the second side (22b) and out of the second Side escaping evanescent field from the pixel array (24) is detectable.

12. A microscope according to claim 11, wherein the predetermined distance (d ₂ ) is smaller than 1.2 microns.

13. A method of generating an image of an object, comprising the steps of:

Exposing the object by arranging a first side (22a) of an optical structure (22, 31; 52) having a negative refractive index (n) at least at a near field distance (di) to the object such that an evanescent field (42) modulated by the object at the first side (22a) is transferred to a second side (22b) of the optical structure (22, 31; 52) opposite the first side; and

Detecting the evanescent field (42) at the second side (22b) by a pixel array (24).

The method of claim 13, wherein the object is illuminated by a monochromatic light (104).

A method according to claim 13 or 14, wherein the first side (22a) of the optical structure (22, 31; 52) is disposed at a distance smaller than 1.2 μm from the object.

The method of any one of claims 13 to 15, wherein the pixel array (24) is at a predetermined distance

(d ₂ ) is arranged along the second side (22b), wherein the predetermined distance of the pixel array (24) is adjusted by the second side (22b) so that an evanescent field transmitted from the first side (22a) to the second side (22b) and exiting from the second side can be detected by the pixel array (24).

17. The method of claim 16, wherein the predefined distance is less than 1.2 μm.

18. An optical device (200) for improving a resolution of diffraction-limited optics, comprising:

a diffraction limited optic (202);

an optical structure (52) having a negative refractive index (n), wherein a first side (22a) of the optical structure (52) is disposed in an image plane of the diffraction limited optical system (202), and wherein the optical structure (52) is adjacent Metal layers (310) having openings (308), the openings (308) having an opening extent, and wherein a lateral offset of the openings (308) of adjacent metal layers (310) is less than the opening extent to provide a solid angle selectivity of the optical structure (52 ) to achieve; and

a pixel array (24) extending at a predetermined distance (d ₂ ) along a second side (22b) of the optical structure (52) opposite the first side (22a).

The optical device of claim 18, wherein the openings (308) of adjacent metal layers (310) lie on a common axis (312) to achieve a solid angle selectivity of the optical structure (52).