CHEMICAL SENSING DEVICE USING FLUORESCENT SENSING MATERIAL

Gas detection by some types of opto-chemical gas sensors is based on a measurement of the fluorescence of a sensing material that changes its optical properties upon exposure to specific gases. Such a gas sensor is provided with a light source, and the sensing material, which may typically be a fluorescent dye, is arranged as a thin layer, so that it can be exposed to the gas and to the light from the light source.

The fluorescence that is displayed by the sensing material upon irradiation is detected by a photodetector. Its

wavelength is usually greater than the wavelength of the exciting light emitted by the light source. There is a variety of optical properties that may be affected by a reaction between the gas and the sensing material, like spectral response, fluorescence intensity or fluorescence lifetime, for example, which allow to identify the gas and typical properties of the gas.

A high sensitivity of the photodetector is important, because only a very small portion of the light that is incident on the sensing material is converted by fluorescence. In order to make the distance between the photodetector and the sensing material as small as possible, the light source should not be placed in between. On the other hand, an illumination of both the sensing material and the

photodetector by the exciting light is to be avoided, as the photodetector is ideally only exposed to the fluorescence. It is an object of the present invention to provide a CMOS- compatible fluorescence-based chemical sensing device.

This object is achieved with the chemical sensing device according to claim 1. Embodiments derive from the dependent claims .

The chemical sensing device comprises a substrate of

semiconductor material, integrated circuit components and a photodetector formed in the substrate, a dielectric on the substrate, a wiring in the dielectric, the wiring providing electric connections for the integrated circuit components and the photodetector, a source of electromagnetic radiation arranged in or above the dielectric, and a fluorescent sensor layer arranged in or above the dielectric. A waveguide is arranged in or above the dielectric. A portion of the

waveguide is arranged at the source of electromagnetic radiation, so that the electromagnetic radiation emitted by the source of electromagnetic radiation is coupled into the waveguide. A further portion of the waveguide is arranged between the photodetector and the fluorescent sensor layer. The source of electromagnetic radiation may be a vertical- cavity surface-emitting laser (VCSEL) or a light-emitting diode (LED), for instance.

An embodiment of the chemical sensing device comprises a grating in the portion of the waveguide that is arranged at the source of electromagnetic radiation, and a further grating in the portion of the waveguide that is arranged between the photodetector and the fluorescent sensor layer. A further embodiment comprises scattering particles at the portion of the waveguide that is arranged between the

photodetector and the fluorescent sensor layer.

A further embodiment comprises a mirror in the portion of the waveguide that is arranged between the photodetector and the fluorescent sensor layer, the mirror being inclined with respect to a direction of propagation of the electromagnetic radiation in the waveguide.

A further embodiment comprises an optical filter arranged between the waveguide and the photodetector. The optical filter blocks the electromagnetic radiation emitted by the source of electromagnetic radiation and is transparent to radiation generated in the fluorescent sensor layer.

A further embodiment comprises a mirror layer arranged between the waveguide and the photodetector. The mirror layer reflects the electromagnetic radiation emitted by the source of electromagnetic radiation.

A further embodiment comprises a carrier, which is

transparent to the electromagnetic radiation emitted by the source of electromagnetic radiation and to radiation

generated in the fluorescent sensor layer. The fluorescent sensor layer is applied on the carrier.

In a further embodiment the fluorescent sensor layer is directly applied on the waveguide or on a cladding layer of the waveguide. In a further embodiment the waveguide and the photodetector are arranged at a distance of less than 10 ym from one another .

The fluorescent sensor layer may especially provide a gas sensor or a bio sensor.

The following is a more detailed description of examples of the chemical sensing device in conjunction with the appended figures .

Figure 1 is a cross section of a chemical sensing device.

Figure 2 is a further cross section of the chemical sensing device according to Figure 1 at the position indicated in Figure 1.

Figure 3 is a cross section of a further chemical sensing

device comprising scattering particles.

Figure 4 is a cross section of a further chemical sensing

device comprising a waveguide mirror.

Figure 5 is a cross section of a further chemical sensing

device having a fluorescent sensor layer directly applied to a waveguide.

Figure 1 is a cross section of a CMOS-compatible chemical sensing device comprising a waveguide. Components of an integrated circuit 2 and a photodetector 3, which may be a photodiode, for instance, are formed in a substrate 1 of semiconductor material, which may be silicon, for instance. A dielectric 4 with embedded metal layers forming a wiring 5 is arranged above the substrate 1. The wiring 5 provides

electric connections for the integrated circuit components 2 and the photodetector 3. The components of an integrated circuit 2, the photodetector 3, the dielectric 4, and the wiring 5 may be formed in a conventional CMOS process, for instance. Figure 1 also shows a contact connection 15 for external electric connection, by way of example.

A source of electromagnetic radiation 6, which may be a vertical-cavity surface-emitting laser (VCSEL) or a light- emitting diode (LED) , for instance, is arranged in or above the dielectric 4. The source of electromagnetic radiation 6 may be monolithically integrated, as shown in Figure 1, or it may be provided by an external device. A waveguide 9 is arranged at the source of electromagnetic radiation 6, so that the electromagnetic radiation emitted by the source of electromagnetic radiation 6 can enter the waveguide 9. The electromagnetic radiation may be coupled into the waveguide 9 by means of a diffraction grating 10, for instance. Butt coupling may be employed instead of the grating 10.

The waveguide 9 may be provided with cladding layers, which may also function as optical confinement of the guided electromagnetic radiation. Cladding or confinement layers of waveguides are known per se and are not represented in the figures. A cover layer 12, which is transparent to the electromagnetic radiation from the source 6 and to the light generated by fluorescence of the sensing material, may be applied to cover the waveguide 9.

The electromagnetic radiation emitted by the source 6 is guided through the waveguide 9 to a region that is provided for sensing. A fluorescent sensing layer 14 is arranged above the photodetector 3. The material of the fluorescent sensing layer 14 may be selected for a gas sensor or a bio sensor. In the embodiment according to Figure 1, the fluorescent sensing layer 14 is applied on a transparent carrier 13, which is optional. Such a carrier 13 may be particularly favourable if an easy replacement of the fluorescent sensor layer 14 is desired after each measurement. The fluorescent sensing layer 14 may instead be applied on the cover layer 12 or immediately on the waveguide 9.

A portion of the waveguide 9 extends between the

photodetector 3 and the fluorescent sensing layer 14. A further grating 11 can be provided in this portion of the waveguide 9 to diffract the electromagnetic radiation out of the waveguide 9 towards the fluorescent sensing layer 14.

The electromagnetic radiation that is released from the waveguide 9 also propagates in the direction towards the photodetector 3. As this is not desired, a monolithically integrated filter layer 7 is arranged between the

photodetector 3 and the waveguide 9. The filter layer 7, which may be an interference filter, for instance, is a band pass filter configured to block the electromagnetic radiation emitted by the source 6. The filter layer 7 is transparent to the electromagnetic radiation generated in the fluorescent sensor layer 14 and thus enhances the detection of the fluorescence in relation to the much stronger exciting light from the source 6.

In order to increase the low conversion rate of the exciting light into fluorescence, a mirror layer 8 may additionally be provided. The electromagnetic radiation that is diffracted from the waveguide 9 towards the photodetector 3 is thereby reflected in the opposite direction towards the fluorescent sensor layer 14. The mirror layer 8 may be a thin metal layer, for instance.

Several different directions of propagation of the

electromagnetic radiation are indicated in Figure 1 with arrows, by way of example. The electromagnetic radiation reaches the fluorescent sensor layer 14 directly from the waveguide 9 or via reflection at the mirror layer 8.

Electromagnetic radiation generated in the fluorescent sensor layer 14 and detected by the photodetector 3 is indicated in Figure 1 by longer vertical arrows.

The use of the waveguide 9 allows to keep the distance between the photodetector 3 and the fluorescent sensor layer 14 small. In particular, the distance between the photodetector 3 and the waveguide 9 may be smaller than

10 ym. The distance between the photodetector 3 and the filter layer 7 may be typically 6.5 ym, and the distance between the filter layer 7 and the waveguide 9 typically 3 ym, for example.

Figure 2 is a cross section of the chemical sensing device at the position indicated in Figure 1. The elements of Figure 2 that correspond to elements shown in Figure 1 are indicated with the same reference numerals. Figure 2 shows two separate waveguides 9 at the position of the further grating 11; the number of individual waveguides in the device is arbitrary. The size of the mirror layer 8, which can be provided for each waveguide 9 separately, can be adapted to the width of the waveguide 9. The mirror layer 8 may only be present below the waveguide 9 and cover only a portion of the area of the photodetector 3, especially less than 50% of its area, for instance. In particular, the lateral extension of the

waveguide 9 may be smaller than the lateral extension of the mirror layer 8, as shown in Figure 2 by way of example. The filter layer 7 may also be provided for each waveguide separately, or it may be formed as an entire layer completely covering the area of the photodetector 3, as in Figure 2.

Figure 3 is a cross section of a further CMOS-compatible chemical sensing device. Elements of the device according to Figure 3 that correspond to elements of the device according to Figure 1 are indicated with the same reference numerals.

In the device shown in Figure 3, scattering particles 16 are arranged at the portion of the waveguide 9 that is present between the photodetector 3 and the fluorescent sensor layer 14. The scattering particles 16, which may be Ti0 ₂ , for instance, scatter the guided electromagnetic radiation out of the waveguide 9. The scattering particles 16 substitute the further grating 11 used in the device according to Figure 1.

Figure 4 is a cross section of a further CMOS-compatible chemical sensing device. Elements of the device according to Figure 4 that correspond to elements of the device according to Figure 1 are indicated with the same reference numerals.

In the device shown in Figure 4, a waveguide mirror 17 is arranged in the portion of the waveguide 9 that is present between the photodetector 3 and the fluorescent sensor layer 14. The waveguide mirror 17 is inclined with respect to the direction of propagation of the electromagnetic radiation in the waveguide 9 and reflects the guided electromagnetic radiation out of the waveguide 9. The waveguide mirror 17 substitutes the further grating 11 used in the device

according to Figure 1. Figure 5 is a cross section of a further CMOS-compatible chemical sensing device. Elements of the device according to Figure 5 that correspond to elements of the device according to Figure 1 are indicated with the same reference numerals.

In the device shown in Figure 5, the waveguide 9 is not covered, and the fluorescent sensor layer 14 is directly applied on the waveguide 9. In order to obtain an exposure of the fluorescent sensor layer 14 to the exciting light from the source 6, the evanescent light field is exploited, and no distortion within the waveguide 9 is required to couple the electromagnetic radiation out of the waveguide 9. In this embodiment the optional mirror layer 8 is of minor importance and therefore omitted in Figure 5.

In a suitable production method for the chemical sensing device, all layers are applied on the substrate that is provided with the integrated circuit components 2 and the photodetector 3. The wiring 5 and the filter layer 7 are arranged in and on the dielectric 4 according to a

conventional back end of line (BEOL) , especially of a

standard CMOS process. Cladding layers and the waveguide are formed, and the fluorescent sensor layer 14 is applied for a gas sensor or a bio sensor.

In an alternative production method, the waveguide 9 and one metal layer provided for the mirror layer 8 are formed on a second substrate, which is then bonded to the substrate 1 comprising the integrated circuit components 2, the

photodetector 3 and the wiring 5. The filter layer 7 can be formed on either substrate and provided with a planarization layer, especially an oxide layer. The described chemical sensing device allows a substantial reduction of the size of a fluorescence-based sensor and considerably increases the amount of detected fluorescence while minimizing the amount of exciting light that reaches the photodetector.

Reference numerals

1 substrate

2 integrated circuit components 3 photodetector

4 dielectric

5 wiring

6 source of electromagnetic radiation 7 filter layer

8 mirror layer

9 waveguide

10 grating

11 further grating

12 cover layer

13 carrier

14 fluorescent sensor layer

15 contact connection

16 scattering particle

17 waveguide mirror