IMAGING

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from US Provisional Patent Application No. 62/505,943 filed May 14, 2017, which is incorporated herein by reference in its entirety.

FIELD

Embodiments disclosed herein relate in general to vision systems in the SWIR wavelength range (1100nm-1900nm), and in particular to vision systems for the automotive industry operating in the 1350nm-1400nm and the 1450-1600 nm wavelength ranges.

BACKGROUND

Current imaging systems operating in the visible wavelength range (400-700 nm) or the near infrared (NIR) regime (typically up to about 1000 nm) are prone to significant reduction in performance in extreme weather conditions, e.g. fog and dust. Furthermore, under high levels of ambient light intensity, such imaging systems may become saturated and cannot perform the task of collecting a high quality image. Typical imaging systems operating at longer wavelengths in the SWIR regime are based on material systems such as InGaAs and are therefore inherently expensive and not suitable for low cost applications.

SUMMARY

Embodiments disclosed herein teach Si-based imaging systems and methods that allow photodetection of light in the SWIR regime. Light in the SWIR band is not visible to the human eye, thus providing an inherent advantage for eye-safe applications. SWIR images are not in color, which makes objects easily recognizable and yields one of the tactical advantages of the SWIR, namely object or individual identification. A large number of applications that are difficult or impossible to perform using visible light are possible using SWIR. When imaging in SWIR, the scattering of light by water vapor, fog and dust particles, for example, are less pronounced and better (e.g. higher signal to noise ratio, or reduced image blur) imaging can be obtained. This is due to the longer wavelength of the SWIR photon with respect to the visible regime. Additionally, colors that appear almost identical in the visible may be easily differentiated using SWIR.

In various exemplary embodiments, there is provided an imaging system, comprising an active illumination source for illuminating a target in a SWIR wavelength range, a FPA of plasmonic enhanced pyramidal silicon Schottky PDs, each PD operative to detect SWIR radiation reflected or scattered from the target and to convert the detected SWIR radiation into an electrical signal, and a ROIC operatively coupled to the FPA and used to read out electrical signals the PDs, wherein the read out electrical signals are convertible into an image of the target. The SWIR wavelength range may include for example the 1350nm-1400nm wavelength range or the 1450nm-1600nm wavelength range. Each plasmonic enhanced pyramidal silicon Schottky PD includes a contact to a silicon side of the Schottky PD and a contact to a metal side of the Schottky PD. In some embodiments, the FPA and the ROIC may be formed integrally in a single silicon wafer. In some embodiments, the FPA and the ROIC may be formed on separate silicon wafers. In some embodiments, FPA may include a plurality of pixels, wherein each pixel includes a single plasmonic enhanced pyramidal silicon Schottky PD. In some embodiments, the FPA includes a plurality of super-pixels, wherein each super-pixel includes a plurality of plasmonic enhanced pyramidal silicon Schottky PDs. In some embodiments with super-pixels, the contact to the metal side may be common to all the Schottky PDs in a super-pixel. In some embodiments and methods of use, an imaging system disclosed herein is included in a vehicle and used in a vehicular environment.

In various exemplary embodiments, there is provided a method comprising actively illuminating a target with radiation in a SWIR wavelength range, using a FPA of plasmonic enhanced pyramidal silicon Schottky PDs to detect SWIR radiation reflected from the target, and converting the detected SWIR radiation into a SWIR image of the target. The FPA may include a plurality of pixels, wherein each pixel includes a single plasmonic enhanced pyramidal silicon Schottky PD, or a plurality of super-pixels, wherein each super-pixel includes a plurality of plasmonic enhanced pyramidal silicon Schottky PDs. BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting examples of embodiments disclosed herein are described below with reference to figures attached hereto that are listed following this paragraph. Identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. The drawings and descriptions are meant to illuminate and clarify embodiments disclosed herein and should not be considered limiting in any way:

FIG. 1 illustrates schematically an imaging system disclosed herein;

FIG. 2 illustrates the operation concept of an embodiment of an imaging system disclosed herein;

FIG. 3 A shows schematically in cross section an embodiment of an integrated imaging system comprising an FPA integrated with a ROIC on a common Si wafer;

FIG. 3B shows schematically in cross section an embodiment of a hybrid imaging system comprising an FPA bonded to a ROIC through a bonding surface;

FIG. 3C shows a super-pixel of 2 x 2 pyramidal PDs;

FIG. 4 shows a calculated SNR as a function of a distance from the active illumination source to the target to be detected.

DETAILED DESCRIPTION

Systems and methods disclosed herein are directed towards enhancing imaging information, particularly in vehicular driving environments, by enabling vision in extreme weather (for example fog, haze, snow, rain, summer) conditions and in all light condition (for example low light or direct sunlight).

FIG. 1 illustrates schematically an embodiment of an imaging system disclosed herein numbered 100. System 100 comprises an active illumination source 102 (e.g. laser, LED or lamp) for illuminating an imaged entity (or "target") and a focal plane array (FPA) 104 of plasmonic enhanced pyramidal silicon Schottky PDs (see also FIGS. 3A- 3C) for receiving and collecting radiation reflected from the imaged entity. FPA 104 is electrically and mechanically coupled to a read-out integrated circuit (ROIC) 106. The ROIC is implemented with the goal of accumulating the electrical signal from each of the Schottky photodetector pixels as an electrical charge and transfer the signal onto output taps for readout. Such a ROIC may be for example implemented in a 3 transistor (3T) or a 4T configurations. Alternatively, one can utilize the fast response time of a nano-sized Schottky diode and construct an ROIC with a sufficiently short RC time constant to support ultrafast operation, e.g. for gated imaging applications.

In an example, FPA 104 and ROIC 106 are integrated in the same Si wafer, FIG. 3A. In an example, the FPA and the ROIC are formed on separate Si wafers, which are then bonded, FIG. 3B. System 100 further includes an illumination lens (or a set of lenses) 108 positioned in an optical path 110 between illumination source 102 and the imaged entity, and collection optics in the form of a detector lens (or a set of lenses) 112 positioned in an optical path 114 between the imaged entity and the FPA. Proper packaging can be used to integrate the collection optics with the FPA. Optionally, system 100 may also include a filter 116 for rejecting all light besides the light emerging from the active illumination source and reflected from the imaged entity, the filter positioned in optical path 114. Active illumination source 102 is driven by a driver 118 and may be energized by an internal or external power source (not shown) through a power connector 120. The active illumination source can be synced to a FPA frequency mode in a well-known way. Signals read out by the ROIC can be transferred to an image processor 122 for processing into a SWIR image of the target. Various functions of the imaging system and its components may be controlled by a controller 124.

In an example, the active illumination source is a ~1350nm LED, having desired monochromatic and illumination characteristics. The optical power of the illumination source can be orders of magnitude higher than that of a light source in the visible range. This, combined with the fact that the wavelength of choice is in the "solar blind" regime, offers a significant advantage in signal to noise ratio (SNR).

FIG. 2 illustrates schematically an exemplary operation concept of system 100 in a vehicular environment. The imaged entity may be for example a vehicle, a pedestrian, a physical barrier or other objects. System 100 is positioned on a vehicle 202 and its active illumination source illuminates various targets, here pedestrians 204 and a vehicle 206. The received/collected radiation is converted into an image of the imaged entity in well-known ways. Full coverage of a scene may be achieved by using a scanning device (not shown) or by integrating several active illumination sources.

FIG. 3 A shows schematically in cross section an embodiment of an integrated imaging system 300, comprising an FPA numbered 302 integrated with a ROIC 304 on a common Si wafer 306. FIG. 3B shows schematically in cross section an embodiment of a hybrid imaging system 300' comprising an FPA numbered 302 bonded to a ROIC 304 through a bonding surface 324. In contrast with all known FPAs operating in the SWIR wavelength range and which are normally based on non-silicon materials such as InGaAs, FPA 302 comprises (as mentioned above) only plasmonic enhanced pyramidal Si Schottky photodetectors 308. In an exemplary embodiment, each PD element of the FPA may have a configuration described in B. Desiatov, I. Goykhman, N. Mazurski, J. Shappir, J. B. Khurgin, and U. Levy, "Plasmonic enhanced silicon pyramids for internal photoemission Schottky detectors in the near-infrared regime," Optica, vol. 2, no. 4, pp. 335-338, (2015). In the embodiments of FIGS. 3A and 3B, FPA 302 integrates single plasmonic enhanced pyramidal Si Schottky PDs into an array, thereby enabling imaging (and not just detection). In an example, the FPA may be rectangular. In an example, the FPA may include hundreds to tens of thousands of plasmonic enhanced pyramidal PDs, each corresponding to a pixel of the FPA. In an example, the FPA may include hundreds to tens of thousands of plasmonic enhanced pyramidal PDs arranged in super-pixels.

By using a configuration of silicon and metals and by allowing the absorption of light to occur in the metal rather than in the silicon, the light at the SWIR wavelength regime can be detected via the process of internal photoemission. Responsivity can be improved as compared to a flat device by constructing an array of pyramidally shaped pixels. Due to the large cross- section of each pyramid, light is collected from a large area that corresponds to a pyramid base 310 thorugh microlenses 316, is concentrated toward a nano apex 312 of the pyramid, is absorbed in the Schottky metal 314 and generates hot electrons. Using an internal photoemission process, the hot electrons cross over the Schottky barrier and are collected as a photocurrent. In an FPA embodiment, each pyramid defines a pixel. In other FPA embodiments, one can define a "super pixel" of several PD pyramids, e.g. in a 2 x 2 PD pyramid arrangement. An example, showing a cross section of a super pixel 330 implemented as a 2 x 2 pyramid structure is shown in Fig. 3C. All the PDs in a super-pixel share a single metal contact 318 (which is connected to all Schottky metals 314). Having more than one pyramid per pixel is advantageous in that it allows to achieve a thinner device while maintaining the lateral dimensions of the pixel fixed.

A variety of metals can be used for the Schottky PDs in order to optimize parameters such as responsivity, signal to noise ratio and manufacturability. The choice of metals can be (but not limited to) e.g. aluminum, copper, a metal silicide, gold, titanium, nickel and more. The availability of the Schottky metal simplifies the process of integration with an ROIC as one of the contacts (e.g. contact metal 318) is directly connected to Schottky metal 314 rather than to the silicon. The other (semiconductor side) contact 320 can be common to all pixels, e.g. by grounding the pyramid array to a ground 322 without the need for two separate contacts per pixel. This way, the challenge of achieving high quality ohmic contacts between the semiconductor and the metal is removed.

As an example, the FPA can include pixels of 5 x 5 μηι with a metal layer covering the top l x l μηι of the pyramid. The FPA is illuminated from its back, through microlenses 316. The top metal is that of the Schottky device, and it can be thick (-200 nm) or thin (down to few nanometers), depending on the available technology.

Example of method of use

In an example of a method of use, light is projected by the active illumination source toward an imaged entity. The signal is scattered from the imaged entity and is collected by the photodetector. By doing so, the existence and the location of the imaged entity (e.g. a vehicle, a pedestrian, a physical barrier or other objects is detected. The wavelength of illumination is around 1350 nm, which matches a region of high atmospheric absorption. As a result, direct sunlight at the specific wavelength band is barely present.

An example calculation of the system performance is described below. Light is emitted from an active illumination source, arrives at a target and is reflected back. Some of the reflected light arrives at a PD. The goal is to find how much light arrives at the detector after being transmitted from the source and reflected/scattered from the target object. This is done by radiometric calculations as follows:

The light intensity emitted by the active source is given by / = ^ = [I /sr] , where

P _L is the optical power of the light source, P _E is the electrical power of the light source, η ₅ is the electrical to optical conversion efficiency of the light source, and Ω is the solid angle.

The optical flux arriving at the target is given by P _L /i4[ ^Watt _; where A is the area of the light spot on the target.

The optical power reflected from the target is W _T = RP _L /A[ ^Watt _; where R is the reflection coefficient of the target. The target is assumed to be Lambertian, i.e. it scatters light at all directions.

The op ½ ^■ Γ

^F tical flux arriving ^& at the PD is given by ^J W _d =——

u 4( _F #)2 ⁱ -m ^W et ^a e ^t r ^t 1

2- ¹ where # is the F- number of the detection optics, given by the ratio between the focal length and the diameter of the lens.

The light power arriving at each pixel is given by P _d = W _d A =,

where A _p is the pixel active area.

A numerical evaluation of the optical power needed to detect a target at distance of about up to 50 meters, with a SNR of 1 is performed next. Assume a target reflectivity of 20%, conventional imaging optics with F#l, FPA pixel size of about 100 microns ² , a field of view (FOV) of 60 xlO degrees. At that distance and with that FOV, the area A of light illuminating the target is about -500 meter . With these parameters, one obtains

- 10

In the SWIR, due to the eye safety advantage, one can easily use a high power light source. To simplify, one can use light source with 250 W CW. Now, assigning P _L = 250I , we obtain P _d = 2.5 * 10 ^~12 W. This power level defines the needed NEP (noise equivalent power) of a PD.

Assuming a shot noise limited PD, the NEP is given by P = ^ ^2eid ₅ where i _d is the dark current

R

and R is the responsivity of the PD. The aim is a quantum efficiency of 30% around the wavelength of 1300 nm, and thus ~0.3A/W . For a lHz bandwidth, the allowed dark current is given by i _d = ^ ^NE ^ ^P = ^ ^Pd* ^— ΙμΑ which is well within the specifications of a Schottky based PD array.

Another way of looking at the problem is by calculating the SNR as a function of distance. FIG. 4 shows the SNR as function of distance from the active illumination source to a target to be detected. The simulation assumes the following parameters: pixel (PD) size 10 xlO μηι, quantum efficiency 10%, dark current density: l e-08 Amp/cm ^A 2, number of pixels 1000 x 250, frame rate 30Hz, F# of the imaging system = 1.4, target reflectivity 20%, wavelength 1400 nm and average laser power of 0.3W. The results show that even under very low power level, one can still detect an object (based on SNR = 1 criterion) from a distance of 100 meters. Clearly, SNR and target range can both be improved by using active illumination source with higher power, as allowed in the SWIR due to more relaxed eye safety regulations as compared with the visible range.

While this disclosure describes a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of such embodiments may be made. In general, the disclosure is to be understood as not limited by the specific embodiments described herein, but only by the scope of the appended claims.

All references mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual reference was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present application.