FR3133239A3 - Infrared filter - Google Patents

Abstract

Infrared filter The present description relates to a filter comprising at least one interference filter (11) and at least one organic filter (13). Figure for abstract: Fig. 4

Description

Infrared filter

The present description generally concerns image acquisition devices or image sensors, and aims more particularly at image acquisition devices based on organic photodetectors.

Infrared filters, rejectors or band cutters of infrared wavelengths, that is to say blocking transmissions through the filter by reflection or absorption, are generally used for fingerprint sensors.

There is a need to improve infrared filtering devices for image acquisition devices.

One embodiment provides a filter, a stack of which comprises:
an interference filter; And
at least one organic filter.

According to one embodiment, the organic filter operates by absorption.

According to one embodiment, the interference filter operates by reflection.

According to one embodiment, the interference filter has an average optical density of the order of 2 in a wavelength range ranging from 600 nm to 900 nm.

According to one embodiment, the organic filter has an average optical density of the order of 1 in a wavelength range ranging from 600 nm to 900 nm.

According to one embodiment, at least one organic filter is above the interference filter.

According to one embodiment, the filter further comprises a red absorbing layer above the interference filter.

One embodiment provides a fingerprint sensor comprising a filter as described.

According to one embodiment, an angular filter is inserted between the interference filter and an organic filter.

According to one embodiment, the sensor comprises:
- a set of photodetectors;
- an angular filter;
- a filter as described; And
- a display screen and/or lighting device.

These characteristics and advantages, as well as others, will be explained in detail in the following description of particular embodiments and implementations made on a non-limiting basis in relation to the attached figures among which:

there represents, partially and schematically, a stack of layers constituting an interference filter;

there illustrates an example of optical density response of the filter of the ;

there represents, partially and schematically, an embodiment of an organic filter;

there illustrates an example of a filter response from the ;

there , there , there , are schematic views illustrating embodiments of image filters made from filters of Figures 1A and 2A;

there represents, in a partial and schematic manner, another embodiment of an image filter;

there represents, partially and schematically, an example of an image sensor integrating a filter as described.

The same elements have been designated by the same references in the different figures. In particular, the structural and/or functional elements common to the different embodiments and implementations may have the same references and may have identical structural, dimensional and material properties.

For the sake of clarity, only the steps and elements useful for understanding the embodiments and implementation described have been represented and are detailed. In particular, the creation of the electronic circuits for controlling and reading the photodetectors of the devices described has not been detailed. In addition, the various applications that the devices described may have have not been detailed.

Unless otherwise specified, when we refer to two elements connected to each other, this means directly connected without intermediate elements other than conductors, and when we refer to two elements connected (in English "coupled") to each other, this means that these two elements can be connected or be linked through one or more other elements.

In the following description, when referring to absolute position qualifiers, such as "front", "back", "up", "down", "left", "right", etc., or relative, such as the terms "above", "below", "superior", "lower", etc., or to qualifiers of orientation, such as the terms "horizontal", "vertical", etc., it is referred to unless otherwise specified in the orientation of the figures.

Unless otherwise specified, the expressions "approximately", "approximately", "substantially", and "of the order of" mean to the nearest 10% or to the nearest 10°, preferably to the nearest 5% or to the nearest 5°.

In the remainder of the description, visible light is electromagnetic radiation whose wavelength is between 400 nm and 700 nm and infrared radiation is electromagnetic radiation whose wavelength is between 700 nm and 1 mm. . In infrared radiation, we distinguish in particular near infrared radiation whose wavelength is between 700 nm and 1.7 μm.

In the remainder of the description, a layer or film is said to be transparent to radiation when the transmittance of the radiation through the layer or film is greater than 10%, preferably greater than 50%.

In some fingerprint sensor applications, the sensors used are sensitive to daylight. This problem is particularly present when operating in an outdoor environment, solar radiation (in the infrared range) disrupting detection by the photodetectors of the fingerprint sensor.

More particularly, we seek to obtain an optical density or absorbance of several units, preferably of the order of 3, or even more, in the near infrared and more particularly in the very near infrared (wavelengths between 600 and 900 nm).

Optical density represents the opposite of the decimal logarithm of transmittance (OD = -Log(transmittance)), with transmittance representing the percentage of rays of a given wavelength λ passing through the sample (the filter)

Fingerprint sensors are generally equipped with infrared rejector filters between a surface (for example a transparent display, an illumination surface or device, etc.) on which one or more fingers of a user are placed and a network of photodetectors.

There represents, partially and schematically, a stack of layers constituting an interference filter 11.

The production of such a filter 11 is in itself usual.

There illustrates an example of optical density response OD of the filter of the .

A filter 11 of the type illustrated in is made up of a stack of successive layers forming an interference filter blocking (reflecting) infrared IR radiation and allowing radiation in the visible V range to pass.

As illustrated by , a multilayer interference filter 11 is relatively efficient (optical density of the order of 2) for filtering (blocking) near infrared IR radiation, in particular very near infrared. However, it also reflects the red R (between 600 and 700 nm) and essentially only allows the blue B and green G components of the visible V to pass.

It is generally not desirable to increase the number of layers to increase the optical density of an interference filter because this generates mechanical stress on the interference filter which can cause cracks.

Furthermore, in certain applications, the reflection of red gives the fingerprint sensor an appearance that is not very pleasant for the user. This reflection is also particularly annoying when the fingerprint sensor is placed under the screen of a mobile phone or display screen. Indeed, the reflection of red radiation harms the quality of the display.

There represents, partially and schematically, an embodiment of an organic infrared filter 13.

There illustrates an example of optical density response OD of the filter of the .

An organic filter 13 is made of a polymer (for example an organic resin) including colored pigments. Such a filter has the properties of being able to selectively absorb certain wavelengths depending in particular on the nature of the pigments used.

An organic filter 13 can therefore have a greater optical density over a range of wavelengths and be chosen to selectively filter the very near infrared. However, as illustrated by , the OD density often has localized peaks, which can be a problem when homogeneous rejection is desired over the 600 to 900 nm range.

In the example of the , it is assumed that the filter 13 has an average optical density of the order of 1 in the very near infrared.

According to the embodiments described, it is planned to add, to an interference infrared filter 11 of the type of that illustrated in Figures 1A and 1B, at least one organic filter 13 of the type of that illustrated in Figures 2A and 2B.

There , there , there , are schematic views illustrating embodiments of filters 1 made from filters 11 and 13 of Figures 1A and 2A.

The addition of an organic filter 13 to an interference filter 11 makes it possible to increase the optical density of the filter 1 and, in particular, to achieve optical densities greater than 3. Indeed, the respective optical densities of an interference filter and one or more superimposed organic filters are added.

The combination of filters 11 and 13 of the type illustrated in Figures 1A, 1B, 2A and 2B therefore makes it possible to overcome the disturbance of solar radiation (wavelengths in the very near infrared) which otherwise influence the operation of the fingerprint sensor. Indeed, we can, by stacking or superimposing these structures, add their respective optical densities.

There represents an example of filter 1 in which an organic filter 13 is located below (in relation to a screen not shown but assumed above the assembly) an interference filter 11.

There represents an example in which an organic filter 13 is located above an interference filter 11.

There represents an example in which two organic filters 13 sandwich an interference filter 11.

Depending on the selectivity and response of the organic filter 13, it can also block (absorb) the red components. In this case, we take advantage of the non-reflective nature of the organic filter 13 when the organic filter 13 is above the interference filter 11 (case of Figures 3B and 3C). It then helps, by absorbing the red reflected by the interference filter 11, to reduce the visual impact for the user.

There represents, partially and schematically, another embodiment of a filter 1 for a fingerprint sensor.

In the embodiment of the , we plan to cover a stack of the type described in , of a layer 15 absorbing wavelengths in the red. The addition of such a layer 15 absorbing red on the upper face of the filter 1 makes it possible to eliminate the reflection caused by the interference filter 11.

Layer 15 is for example a non-adhesive layer. For example, it is a resin incorporating colored pigments (red). In this case, the organic filter 13 provides the additional optical density (of the order of 1) to the interference filter 11 so as to achieve an optical density of the order of 3. However, as the red component is reflected by the filter 11 and not captured by the filter 13 which is located below, we add a layer 15 with red pigments for the visual comfort of the user (the rendering of the display is no longer modified).

In certain applications, the filter integration process requires providing an adhesive layer under the interference filter 11 to stick the filter to an image sensor or another optical filter. If necessary, a non-adhesive finishing layer (buffer) is also provided, for example to provide an air gap between the filter 1 and a display screen.

There represents, partially and schematically, an example of an image sensor integrating a filter 1 as described.

The filtering structure 1 described in relation to the preceding figures is preferably associated with an image sensor made up of a network of photodetectors.

For example, a stack of the type described in relation to Figures 3A, 3B, 3C and 4 can be inserted between an angular filter 3 surmounting an array of photodetectors 2 and a display screen 4 or a lighting device.

According to an alternative embodiment, the angular filter 2 is inserted between the interference filter 11 and the organic filter 13. It is then integrated into the filter 1.

Various embodiments and variants have been described. Those skilled in the art will understand that certain features of these various embodiments and variants could be combined, and other variants will become apparent to those skilled in the art. In particular, the cut-off wavelengths of the interference filters taken as an example are of the order of 600 to 900 nm considering that the photodetectors of the fingerprint sensor are not sensitive beyond that (and are therefore not disturbed by higher wavelengths). These wavelengths could nevertheless be adapted if the photodetectors are sensitive to higher infrared wavelengths.

Finally, the practical implementation of the embodiments and variants described is within the reach of those skilled in the art based on the functional indications given above.

Claims (10)

Filter (1) of which a stack comprises:
an interference filter (11); And
at least one organic filter (13). Filter according to claim 1, in which the organic filter (13) operates by absorption. Filter according to claim 1 or 2, in which the interference filter operates by reflection. Filter according to any one of claims 1 to 3, in which the interference filter (11) has an average optical density (OD) of the order of 2 in a wavelength range ranging from 600 nm to 900 nm. Filter according to any one of claims 1 to 4, in which the organic filter (13) has an average optical density (OD) of the order of 1 in a wavelength range ranging from 600 nm to 900 nm. Filter according to any one of claims 1 to 5, in which at least one organic filter (13) is above the interference filter (11). Filter according to any one of claims 1 to 6, further comprising a red absorbing layer (15) above the interference filter (11). Fingerprint sensor comprising a filter according to any one of claims 1 to 7. Sensor according to claim 8, in which an angular filter (3) is interposed between the interference filter (11) and an organic filter (13). Sensor according to claim 8 or 9 comprising:
- a set of photodetectors (2);
an angle filter (3);
a filter according to any one of claims 1 to 9;
a display screen and/or lighting device (4).