Patent ID: 12249669

DESCRIPTION OF EMBODIMENTS

FIG.1schematically shows a microscopic system1as an exemplary application for a photodetector array. The microscopic system1has an optical objective2with a high numerical aperture to collimate light from a sample3into the microscope body4. The collimated light L is projected onto a photodetector array5by means of a focusing tube lense6having a low numerical aperture. Further lenses6aare included to cross light L on a pinhole8. Illumination is made by an illumination device7which couples illumination light in onto the sample3by means of a semitransparent mirror9.

For microscopy applications, a high sensitivity is required with a low noise, a low dark count rate, a high dark count rate uniformity and a low cross-talk between the neighboring pixels. For image-scanning microscopy which enables a theoretical image resolution improvement by a factor of 2 and an improved light collection, a fast photo-counting photodetector array with less than 1 μs integration time is required.

As the illumination intensity has to be restricted to reduce phototoxicity and photobleaching, a very sensitive detector array5is required. The most sensitive photodetector arrays use as photodiodes single-photon avalanche diodes, which have a photon counting ability and a high signal-to-noise ratio.

In the following, the photodetector array5is described using photodetector cells each provided with a single-photon avalanche diode.

FIG.2shows a top view on an arrangement of photodetector cells11of an embodiment of a photodetector array10. The photodetector cells11are arranged in a semiconductor substrate as known in the art. The photodetector cells11each form a pixel of the photodetector array10and comprise single-photon avalanche diodes (SPAD)12. The photodetector cells11are arranged in a hexagonal grid and therefore have a staggered arrangement which allows to compact the photodetector cells11with its small active areas13arranged in the center of the photodetector cells11.

Particularly, the portion of the active area13of the photodetector cells compared to the area of the photodetector cell11is between 5 to 33%. This allows a larger distance between the active areas13of the photodetector cells so that the cross-talk between active areas13of neighboring photodetector cells11can be kept quite low.

Further, the relatively small active area13is beneficial as the photodiode capacitance and the charge flow through the photodiodes is significantly reduced.

The reduction of the photodiode active area13requires a large recovery of the fill factor by means of microlenses14for each of the photodiodes to maintain or to increase the sensitivity of the photodetector cells11of the photodetector array10. The microlenses14may be formed as a single microlens layer on top of the semiconductor substrate. The area of light focused by the microlenses14which lies within the active area13is indicated by16.

FIG.2shows the arrangement of the photodiodes with a circular active area13in a total photodetector cell area which is hexagonal or circular wherein on top of each of the photodetector cells11a microlens14is arranged. The microlenses14therefore also have a staggered arrangement corresponding to the arrangement of the photodetector cells11. In difference to the active areas13of the photodetector cells11, the microlenses14do not need to have to suppress optical or electrical interference. Therefore, none or only a small gap15between neighboring microlenses14may be provided.

InFIGS.3aand3b, cross-sectional views of two embodiments for the arrangement of two neighboring photodetector cells11are shown. The arrangement shows two neighboring photodetector cells11with its semiconductor depletion region divided by a deep-trench isolation22in between. On the surface S of the semiconductor substrate20, a microlens array21is formed having a residual height R with a dome-shaped microlens14on top having a sag height H and being distanced from neighboring dome-shaped microlens14by the gap15. This allows to form a focusing microlens14which collects and directs incoming photons onto the active area of the respective photodetector cell11underneath.

FIG.3bshows a similar design, wherein between the microlenses14a reflective material17is included. Furthermore, the deep-trench isolation22may be made by a reflective material so that incoming photons may be reflected once or several times on the sidewalls formed by the reflective material.

FIG.4ashows the effective fill factors for circular and square microlenses used with pixels with 1 μm active area radius and a native fill factor of 0.7% and 0.6% for a hexagonal and square grid of pixels, respectively. One can observe a much higher effective fill factor for circular microlenses on a hexagonal grid when compared to the square microlenses on a square grid.FIG.4ashows the curves for two f numbers indicating different collimation of incident light.

FIG.4bshows the effective fill factors for circular and square microlenses used with pixels with 3 μm active area radius and a native fill factor of 6.7% and 5.8% for a hexagonal and square grid of pixels, respectively.

FIG.4cshows the effective fill factors for circular and square microlenses used with pixels with 6 μm active area radius and a native fill factor of 27% and 23.4% for a hexagonal and square grid of pixels, respectively.

As it is clear fromFIG.4bandFIG.4c, the circular microlens on a hexagonal grid is also more robust to variations in microlens residual height. An active area radius of less than 3 μm is not desirable due to lateral shifts of microlenses with respect to the photodetector substrate. Lateral shifts decrease the robustness to microlens residual height variations.

The staggered arrangement of circular microlenses yields for a fill factor of 90.6% compared to a fill factor of 100% of squared microlenses of a rectangular arrangement. However, the photon loss of the rectangular-shaped microlenses is high, so that the staggered arrangement of circular or hexagonal microlenses14effectively directs more photons onto the active area13of the photodetector cell11than rectangular-shaped microlenses. Furthermore, circular or hexagonal microlenses turned out to be more robust with respect to lateral shifts and variations in microlens residual height R occurring in the microlens production, as clear fromFIG.4.

The photodetector active area13is provided with a circular shape which may reduce the edge electrical field compared to squared active areas. Preferably, the radius of the active area13of the photodetector cell11is smaller than 9 μm, more preferred smaller than 5 μm. The smaller the active area13is, the lower is the chance to capture impurities in the semiconductor material which further reduces the count of noisy pixels.

In contrast thereto, the size of the active area13of the photodetector cell should be not smaller than 3 μm due to a lateral shift variations for the placement of microlenses on top of the photodetector array10as well as due to microlens residual height variations.

The substantial area difference between the active area13and the photodetector cell11allows to accommodate the pixel electronics around the active area13which is composed of a quenching transistor, a gate for capacitive isolation of the photodiode, an active recharge mechanism and a photodiode disabling memory. The gate for capacitive isolation of the photodiode allows to have a capacitive isolation to reduce the cross-talk and after-pulsing due to a smaller amount of charge flowing through the photodiode. The quenching transistor and the gate for capacitive isolation need to be positioned as close as possible to the photodiode output contact so that the connections do not increase the capacitance significantly. The electronics for active recharge mechanism and a photodiode disabling memory can be placed farther away from the photodiode output contact since an increased capacitance do not affect the performance greatly.

As further shown inFIG.2, the metal connections extend in a rectangular fashion between the substrate surface and the microlens layer. The read out lines18run along straight lines along the arrangement direction of the photodetector cells11between two neighboring rows of photodetector cells11, respectively. The power lines19substantially extend perpendicular to the read out lines but are guided in a serpentine way around the active areas13of the photodetector cells11.

Since high-performance applications require to read all photodetector signals in parallel within a small integration time and without delays between the read-outs of the individual photodiodes, each photodiode needs to have a single connection to the outside of the array. These connections should be as short as possible to enable high-speed and low-jitter data transmission so that read out lines18are used for the photodiode outputs, while the serpentine power lines19are used for the power supply of the photodetector cells11.