Patent Description:
Electromagnetic radiation, e.g. including waves of ultraviolet light, visible light or infrared light, may be useful as information carriers in many different applications. Various detectors are therefore employed to record the electromagnetic radiation and transform it into an electric signal that can be processed and analysed.

In imaging devices, such as for example cameras, electromagnetic radiation in the visible wavelength range may be recorded by means of a CMOS sensor comprising colour filters. The pixels of the camera may comprise colour filters that are employed to filter out different wavelength ranges of the incoming light, such as for example red, green and blue light, and direct the individual colours to a respective sensor. The registered signal from each sensor may then be used to determine the intensity of the respective colour and thus the wavelength composition of the incoming light.

This technology has however turned out to be associated with drawbacks, in particular regarding the prospects of further downscaling. The filtering of the incoming light reduces the sensitivity of the sensor, and problems have arisen regarding optical cross talk and reduced production yield.

In order to improve detection of electromagnetic radiation, it would be of interest to increase the sensitivity of the detector and enable further downscaling.

<CIT> discloses a device comprising a substrate having a front side and a back-side, a nanowire disposed on the back-side and an image sensing circuit disposed on the front side, wherein the nanowire is configured to be both a channel to transmit wavelengths up to a selective wavelength and an active element to detect the wavelengths up to the selective wavelength transmitted through the nanowire.

An objective of the present inventive concept is to provide a semiconductor detector exhibiting a high sensitivity to electromagnetic radiation impinging on the detector. It is a particular objective of the present inventive concept to provide a detector that utilises a first and a second waveguide portion for guiding electromagnetic radiation towards a photodetector in a manner that allows for a separate detection of the electromagnetic radiation transmitted through the first waveguide portion and the second waveguide portion.

These and other objectives of the invention are at least partly met by the invention as defined in the independent claim.

The present invention is associated with several advantages. Firstly, by out-coupling electromagnetic radiation of the sub-range from the first waveguide portion, a colour-splitting of the incoming light is enabled, which in turn allows for a sensitivity enhancement of the detector. The sensitivity may in particular be increased in relation to other technologies utilising colour filters to filter out different wavelengths from the incoming light. With the present invention, it is possible to discriminate different wavelengths without using colour filters for controlling the wavelengths reaching the photoactive layer.

Secondly, the arrangement of the first and second waveguide portions along the first direction, which may be referred to as a vertical arrangement, allows for the waveguide portions to be manufactured with a cross section (taken across the first direction) having a width in the order of the wavelength of the light that is to be detected, i.e., in the range of hundreds of nanometres. This allows for the incoming electromagnetic radiation to be detected with a resolution that corresponds to the wavelength of the electromagnetic radiation.

Thirdly, the photodetector may be arranged directly beneath the waveguide portions, as seen in the direction of transmission of the electromagnetic radiation, which allows for a near-field arrangement that enables further downscaling of the detector.

The term "waveguide" may generally be understood as an element capable of transmitting electromagnetic waves along its length direction, which in the context of the present application is referred to as "first direction". Thus, the semiconductor detector according to the present invention is oriented along the propagation direction of the electromagnetic radiation, with the funnel element and the photodetector arranged at opposite ends of the first waveguide portion, such that the incoming electromagnetic radiation is guided from the funnel element to the photodetector along the length direction of the first waveguide portion.

The first waveguide portion may be referred to as a "full spectrum" waveguide, which should be understood as a waveguide capable of guiding a broader wavelength range than the second waveguide portion. Preferably, the first waveguide portion is a full spectrum waveguide in the sense that it, during use, may transmit electromagnetic radiation within the visible wavelength range. The first waveguide portion may be designed in relation to the electromagnetic waves to be guided by the first waveguide portion. Dimensions of the first waveguide portion need hence not be limited for use with a single specific wavelength of the electromagnetic radiation. Rather, the first waveguide portion may be used with a range of wavelengths, while providing desired propagation properties.

The second waveguide portion may be referred to as a "colour splitting" waveguide, referring to its capability of out-coupling electromagnetic radiation within a sub-range of the wavelength range of the electromagnetic radiation received by the first waveguide portion. Thus, the second waveguide portion may be understood as a waveguide capable of sorting out certain colours of incoming visible light, such as for instance a sub-range of wavelengths corresponding to red, green or blue light. Similar to the first waveguide portion, the second waveguide portion may hence be designed in relation to the electromagnetic waves to be guided by the second waveguide portion. Dimensions of the second waveguide portion need therefore not be limited for use with a specific wavelength of the electromagnetic radiation, but rather a sub-range of wavelengths corresponding to for example a red, green or blue spectrum while providing desired propagation properties.

The first and second waveguide portions are arranged in parallel and beside each other, as seen in a direction orthogonal to the general propagation direction of the guided electromagnetic radiation. Preferably, they are aligned with each other such that their respective end portions, through which the guided electromagnetic radiation exits the first and second waveguide portions, are arranged on, or at the same distance from, the photodetector. It is preferred to arrange the end portions of the waveguide portions as close to the photodetector as possible, or even in an abutting manner, so as to provide a near-field configuration and allow for further downscaling of the detector.

As a consequence of the second waveguide portion being able to out-couple only a sub-range of the wavelength range guided through the first waveguide portion, the second waveguide portion may be dimensioned differently from the first waveguide portion. The operational wavelength range of the second waveguide portion may be determined by its cross section, i.e., the dimensions of the waveguide portion in a plane orthogonal to the propagation direction of the electromagnetic radiation through the first waveguide portion.

The coupling of electromagnetic radiation between the first waveguide portion and the second waveguide portion may be determined by the interface between the two portions. Preferably, the second waveguide portion is shorter than the first waveguide portion, and arranged such that the length portion of the first waveguide portion arranged between the second waveguide portion and the funnel element fulfils a single mode requirement for the sub-range that is to be out-coupled to the second waveguide portion. Thus, the coupling efficiency and the desired wavelength sub-range to be coupled may determine the dimensioning and relative orientation of the first and second waveguide portions.

According to the claimed invention, the semiconductor device comprises a funnel element arranged above the first waveguide portion, such that the first waveguide portion is stacked between the funnel element and the photodetector. The funnel element is arranged to receive incoming electromagnetic radiation via an upper surface of the funnel element and transmit the light into the first end of the first waveguide portion via a lower portion, arranged opposite the upper surface. The upper surface has a larger area than a cross section of the lower portion of the funnel element, such that the incoming light is funnelled into the first waveguide portion.

A single funnel element may be configured to transmit incoming light into a plurality of first waveguide portions. Further, it is appreciated that the funnel element may be formed as a separate part that is connected to the first waveguide portion, or as a unitary item formed in one piece with the first waveguide portion.

The photodetector is understood as a means capable of transforming incident electromagnetic radiation, such as visible light, into an electrical signal. The photodetector may comprise in addition to the photoactive layer, a contact layer and an interconnect layer arranged on a semiconductor substrate. The photoactive layer is arranged for converting photons into an electrical current, and may preferably be arranged closest to the waveguide portions. The contact layer may be arranged in between the waveguide portion and the photoactive layer. The interconnect layer may be arranged between the photoactive layer and the substrate. The contact layer and the interconnect layer may comprise electrodes to conduct the electric current to electric circuits and devices required for reading and processing the electric signal. The contact layer may be un-patterned, extending as a uniform layer in between the waveguide portions and the photoactive layer. The photoactive layer may be patterned into separate regions, for example corresponding to each of the waveguide portions, or be formed as a uniform layer extending below the waveguide portions. In some embodiments, the interconnect layer may be patterned into a plurality of electrodes or electrode regions, which for example may be aligned with a respective one of the first and second waveguide portions. The interconnect layer may hence comprise a first electrode associated with the first waveguide portion, and a second electrode associated with the second waveguide portion. The first electrode may be arranged below the first waveguide portion to register electromagnetic radiation guided through the first waveguide portion, whereas the second electrode may be arranged below the second waveguide portion to register electromagnetic radiation guided through the second waveguide portion.

Further, the first and second electrodes, and/or the portion of the photoactive layer arranged between the first and second electrodes and their respective waveguide portion, may be provided with thickness that is individually adjusted to the specific wavelength range to be detected. Hence, the electrode and/or photoactive layer portion under the first waveguide portion may be thicker and the electrode and/or photoactive layer portion under the second waveguide portion, and vice versa. This may advantageously improve the overall performance of the photodetector.

Reference may herein be made to a "vertical" direction to denote a direction along the first direction, or a normal to a substrate supporting the photodetector (i.e. a normal to a main/upper surface of the photodetector). Meanwhile, "vertical" qualifiers such as "below" and "above" may be used to refer of relative positions with respect to the vertical direction, and do hence not imply an absolute orientation of the detector. Accordingly, the term "below" may be used to refer to a relative position closer to a main surface of the substrate. The term "above" may be used to a position farther from a main surface of the substrate. For example, a first level or element located below a second level or element implies that the first level or element is closer to the main surface of the substrate than the second level or element is. Conversely, a first level or element located above a second level or element implies that the first level or element is farther from the main surface of the substrate than the second level or element is.

The term "horizontal" may meanwhile be used to denote a direction or orientation parallel to the substrate (i.e. a main plane of extension or main surface thereof), or equivalently transverse to the vertical direction. Further, a lateral direction may be understood as a horizontal direction.

The first waveguide portion and the second waveguide portion form separate waveguides. This is to be understood as the waveguide portions being made of two separate pieces, or being provided in the detector as two separate pieces that either are attached to each other or arranged spaced apart. Even though it may be advantageous to arrange the waveguide portions as closely together as possible, preferably with a zero separation, the waveguide portions may be arranged with a lateral spacing of <NUM> or less.

According to an embodiment, the waveguide portions may be arranged directly on the photodetector, such that electromagnetic radiation that is guided through the first and/or second waveguide portion may be transmitted into the photodetector without passing any intermediate layer or spacing. In case the photodetector comprises a contact layer, such as for example an optically transparent and electrically conducting layer comprising for instance indium tin oxide, ITO, the end portions of the first and second waveguide portions may be arranged directly on the contact layer. As a consequence, the waveguide portions may be arranged directly on the photodetector, but separated from the photoactive layer of the detector by a distance corresponding to the thickness of the contact layer.

Thus, the waveguide portions may according to some embodiments be arranged such that they are separated from the photoactive layer by a distance, which preferably may be smaller than a wavelength lying within the sub-range of the wavelength range of the electromagnetic radiation.

According to an embodiment, at least one of the second end of the first waveguide portion and the end of the second waveguide portion may have a funnel shape. In other words, the first waveguide portion and/or the second waveguide portion may comprise a funnel shaped end portion funnelling the guided electromagnetic radiation into the detector. The funnel shape may be help to concentrate the guided electromagnetic radiation to certain regions of the detector, which may make it easier to discriminate the electromagnetic radiation passing through the first waveguide portion from the electromagnetic radiation passing through the second waveguide portion at the detector and to reduce the risk for cross-talk.

According to some embodiments, a waveguide portion (i.e., the first and/or second waveguide portion) may have a substantially constant cross section along a major portion of its length extension. Put differently, the waveguide portion may have sidewalls that are oriented parallel to the first direction, or orthogonal to the main surface of extension of the detector. However, according to some embodiments, the waveguide portion may have a tapering shape, such that a cross section of the waveguide portion is reduced towards the detector. In other words, the sidewalls of the waveguide portion may form a non-orthogonal angle with the main surface of extension of the detector, or be oriented non-parallel to the first direction. A tapering shape may be particularly advantageous for waveguide portions having a relatively high index of refraction compared to the surrounding material, since this may improve optical confinement and increase the efficiency of the waveguide portion's capability of guiding electromagnetic radiation. In an example, the waveguide portions may be formed of silicon nitride and the surrounding material of silicon.

According to an embodiment, the tapering angle, i.e., the angle between the sidewall of the waveguide and the main plane of extension of the detector, may be <NUM>°.

In the drawings, like reference numerals will be used for like elements unless stated otherwise.

Referring now to <FIG>, a semiconductor detector <NUM> for detecting electromagnetic radiation within a wavelength range will be described.

The detector <NUM> is schematically illustrated in <FIG> in a perspective view. The detector <NUM> in the present example utilises a plurality of first waveguide portions <NUM>' and second waveguide portions <NUM>' for guiding incident electromagnetic radiation from a funnel element <NUM> towards a photodetector <NUM>.

Each one of the first waveguide portions <NUM> may be arranged to extend in a first direction, such as a vertical direction, and may further be configured to be a single-mode waveguide for electromagnetic radiation within the wavelength range. In the following, the present invention will be exemplified with a detector for visible light, thus having a wavelength range in the visible part of the spectrum. Other wavelength ranges are however possible as well, such as for example infrared or ultraviolet, and the present invention should therefore not be construed as limited to visible light only.

Each one of the first waveguide portions <NUM>' may extend between the funnel element <NUM>, which may be provided as a separate element for each of the first waveguide portions <NUM>, or form part of a larger structure that is common for several waveguide portions as shown in the present figure, and the photodetector <NUM>, such that incoming light can be guided from a receiving upper surface <NUM> of the funnel element <NUM> towards the photodetector <NUM>.

A cross section of a first waveguide portion <NUM>, taken across the length direction or vertical direction of the first waveguide portion <NUM>', <NUM>" may be substantially constant along the entire length of the waveguide portion <NUM>. Put differently, the sidewalls of the waveguide portion <NUM> may be substantially vertical, or parallel to a normal to a main plane of extension of the photodetector. Such a configuration is illustrated in <FIG>. However, it is appreciated that other configurations may be employed as well, such as for example a tapered shape in which the cross section of the first waveguide portion <NUM>' is reduced towards the photodetector <NUM>. The tapering may be characterised by the angle the sidewall forms with the main plane of extension of the photodetector <NUM>. In the present figure, the sidewalls may form a <NUM>° angle with the photodetector. In other embodiments, not illustrated in the present figure, one or several sidewalls of the first waveguide portion <NUM> may form an angle that is less than <NUM>°, such as for example <NUM>° or less. The choice between straight sidewalls (i.e., constant cross section) or tapered sidewalls may depend on the desired optical transmission properties, which in turn may be determined by the type of material of the waveguide, the wavelengths of the electromagnetic radiation, and the desired coupling to the second waveguide portion <NUM>.

Each of the second waveguide portions <NUM> illustrated in <FIG> extends along the first direction in parallel with the first waveguide portion <NUM>, and is configured to out-couple light within a sub-range of the wavelength range of the light in the first waveguide portion <NUM>. In the present example, the second waveguide portions <NUM> may be configured to couple out for example red, green or blue light to allow those colours to be detected separately at the photodetector. Thus, by out-coupling a sub-range corresponding to for example red, green or blue light, a contrast in signal may be achieved between the light transmitted through the first waveguide portion <NUM> and the second waveguide portion <NUM>.

The second waveguide portions <NUM> may, similarly to the first waveguide portions <NUM> be dimensioned with straight sidewalls (as illustrated in <FIG>) or by one or several tapered sidewalls. The cross sectional shape may be determined by the wavelengths that is to be out-coupled.

As indicated in the present figure, the second waveguide portions <NUM> may be arranged slightly spaced apart from the first waveguide portions <NUM>, such that they are separated by a vertical gap extending along the length direction of the waveguide portions. Other configurations are however possible, in which the first and second waveguide portions are arranged in contact with each other.

Preferably, the first waveguide portion <NUM> and/or the second waveguide portion <NUM> and, optionally, the funnel element <NUM> may be at least partly surrounded or embedded in a material having a lower refractive index than the material of the waveguide portions. Examples of such materials/surroundings may include vacuum, air, and silicon dioxide.

The first and second waveguide portions <NUM>, <NUM> and the funnel element <NUM> may be formed of a material comprising silicon, such as for instance silicon nitride. For visible light applications, the upper surface <NUM> of the funnel element <NUM>, through which the incoming light enters the detector <NUM>, may for example have a size of about <NUM> x <NUM>, and may further be configured to funnel the light into the first waveguide portion <NUM> having a cross section of about <NUM> x <NUM>. In the present example of <FIG>, the total length of the first waveguide <NUM> and the funnel element <NUM> may be about <NUM>.

Depending on the dimensions of the coupling region, i.e., the interface between the first and second waveguide portions <NUM>, <NUM>, light within the sub-range may be coupled into the second waveguide portion <NUM> and guided towards the photodetector arranged at the lower end portions <NUM>, <NUM> of the first and second waveguide portions <NUM>, <NUM>, respectively. The waveguide portions <NUM>, <NUM> may be arranged directly on the photodetector <NUM> such that the end surfaces, through which the guided light may exit the waveguide portions <NUM>, <NUM>, are in direct contact with the photodetector <NUM>.

In <FIG> a photodetector <NUM> according to an embodiment is disclosed. The photodetector comprises a photoactive layer <NUM> configured to respond to photons photoelectrically, thereby allowing the guided light to form an electric signal that can be measured. The photoactive layer may for example be a layer of amorphous silicon, germanium, or perovskite.

The photoactive layer <NUM> may be covered by a contact layer <NUM>, such as instance an optically transparent and electrically conducting layer arranged between the photoactive layer and the first and second waveguide portions <NUM>, <NUM>. The contact layer <NUM> may for example comprise indium tin oxide, ITO.

The photoactive layer <NUM> may be provided above a substrate <NUM>, such as for example a silicon substrate <NUM>, being a front-end-of-line substrate comprising transistors and other devices for handling the electric signal generated by the guided light. Further, an interconnect layer <NUM> may be arranged between the photoactive layer <NUM> and the substrate <NUM> for transmitting the signal between the two. As indicated in the present figure, the interconnect layer <NUM> may comprise a plurality of electrodes, each of which being arranged to contact the photoactive layer <NUM> at positions corresponding to the positions in which the guided light is transmitted to the photoactive layer <NUM> by the waveguide portions <NUM>, <NUM>. Preferably, the electrodes are arranged as close as possible to the waveguide portions <NUM>, <NUM>, such as <NUM> or less.

<FIG> illustrates an example comprising six pairs of first and second waveguide portions <NUM>, <NUM>. Two types of colour splitting waveguides, i.e., first waveguide portions, are used - a first type <NUM>' for red light, and a second type <NUM>" for blue light - whereas the first waveguide portions <NUM> are used for green light. With this configuration, each pair of first and second waveguide portions may be capable of providing a contrast in signal between green light and either red or blue light.

The embodiment of <FIG> shows waveguide portions <NUM>, <NUM> having a substantially uniform cross section along their length direction. Other configurations are however also possible, in which the cross section may vary along the length of the waveguide portions.

<FIG> shows a further embodiment, in which a plurality of second wavelength portions <NUM> may be used in a detector that otherwise may be similar to the detector discussed in relation to <FIG>. In the present example, the detector <NUM> comprises a first waveguide portion <NUM> extending between a funnel element <NUM> and the photodetector (not shown in figure <NUM>). The plurality of second waveguide portions <NUM>, or colour splitting waveguides, may be arranged at one or several sides of the first waveguide portion <NUM> in order to enable more specific wavelengths to be separately detected. By arranging the second waveguide portions <NUM> in an array, in which light is allowed to be coupled between neighbouring ones of the second waveguide portions <NUM>, it is possible to differentiate between a plurality of different wavelength sub-ranges. As discussed in connection with the previous embodiments, the second waveguide portions <NUM> may either be provided as separate structures that are arranged spaced apart or in contact with each other (and, possibly, the first waveguide portion <NUM>).

Claim 1:
A semiconductor detector for electromagnetic radiation within a wavelength range, comprising:
a first waveguide portion (<NUM>) extending in a first direction and configured to be a single-mode waveguide for electromagnetic radiation within the wavelength range;
a funnel element (<NUM>) configured to funnel incident electromagnetic radiation into a first end (<NUM>) of the first waveguide portion;
a second waveguide portion (<NUM>) extending along the first direction and parallel with the first waveguide portion, and being configured to selectively guide electromagnetic radiation within a sub-range of the wavelength range;
wherein the first waveguide portion and the second waveguide portion are separate waveguides;
wherein said second waveguide portion is coupled to the first waveguide portion and configured to out-couple electromagnetic radiation from the first waveguide portion, within the sub-range; and
a photodetector (<NUM>) including a photoactive layer (<NUM>) arranged at a second end (<NUM>) of the first waveguide portion and at an end (<NUM>) of the second waveguide portion, and configured to separately detect electromagnetic radiation transmitted through and exiting the first waveguide portion and the second waveguide portion in the first direction.