WO2018024419A1

WO2018024419A1 - Pyroelectric sensor for detecting electromagnetic radiation

Info

Publication number: WO2018024419A1
Application number: PCT/EP2017/066448
Authority: WO
Inventors: Ingo Herrmann; Fabian Henrici
Original assignee: Robert Bosch Gmbh
Priority date: 2016-08-03
Filing date: 2017-07-03
Publication date: 2018-02-08
Also published as: DE102016214296A1

Abstract

The invention relates to a method for measuring electromagnetic radiation, in particular thermal radiation, with a pyroelectric sensor (1), and the invention relates to a pyroelectric sensor (1) for carrying out the method, wherein the pyroelectric sensor (1) has a pixel array comprising a multiplicity of pixels (10), which are mounted movably over a substrate material (11), and wherein the pixels (10) have a pyroelectric layer (12), which is formed at least partly between a bottom electrode (13) and a top electrode (14) of the pyroelectric sensor (1), wherein the method comprises at least the following steps: irradiating the pyroelectric layer (12) with an electromagnetic radiation, establishing at least indirect contact between the pixel (10) and the substrate material (11) by moving the pixel (10) up to the substrate material (11) from a starting position so as to produce a thermal short-circuit between the pixel (10) and the substrate material (11), reading out a charge transfer in the pyroelectric layer (12) via the bottom electrode (13) and the top electrode (14) and returning the pixel (10) to the starting position away from the substrate material (11).

Description

description

title

Pyroelectric sensor for the detection of electromagnetic radiation

The invention relates to a method for measuring electromagnetic radiation, in particular heat radiation, with a pyroelectric sensor, wherein the pyroelectric sensor has a pixel array with a plurality of pixels, which are movably received on a substrate material, and wherein the pixels have a pyroelectric layer, the at least is formed in sections between a bottom electrode and a roof electrode of the pyroelectric sensor. Furthermore, the invention relates to a pyroelectric sensor for carrying out the method.

STATE OF THE ART

DE 697 36 225 T2 shows by way of example a sensor for detecting electromagnetic radiation, in particular heat radiation, comprising a pixel array with a plurality of pixels, the pixels having a substrate material and an absorber material which is arranged on the substrate material. In this case, several exemplary embodiments are shown in which geometric configuration the pixels can be embodied.

EP 0 716 293 A1 discloses a radiation detector with a sensitive bimetallic cantilever arm connected at one end to a radiation absorbing membrane and a reference cantilever arm which may also be bimetallic. Under the influence of incident radiation on the membrane, the free end of the sensitive arm, due to its heating, approaches the free end of the reference arm to alter the electrical capacitance between the two free ends. A basis of the principle as well as further examples of radiation detectors using an element consisting of two metals is shown in US Pat. No. 3,415,712 A. The known designs of the pixels have in common that they do not or only partially for arrangement in a very small pixel array for the construction of microbolometers are suitable and can be tilted by the suspension of the absorber over the substrate material, whereby the measurement of the heat radiation and thus the imaging quality of the thermal sensor is deteriorated. Microbolometers work alternatively based on diodes and are also known from the prior art, see for example DE 697 16 546 T2.

The principle of the microbolometer based on diodes based on the fact that temperature-sensitive components are provided in a series circuit of, for example, four diodes. The measured variable forms the differential voltage of two pixels with identical current supply. A first pixel is an active pixel, which is characterized by a large thermal insulation and thus a high temperature swing upon irradiation. The second pixel is thermally much better connected to the substrate and serves to compensate the self-heating by the current itself and the compensation of the high offset as a result of the chip temperature. This compensation is necessary because the actual measurement signal is only a microvolt to a millivolt at an offset of about 2.5 volts.

Bolometers based on the bimetallic effect with optical evaluation and laterally next to the pixel arranged bimetallic and Schichtstresskompensations- structures are also known. The disadvantage here is a low fill factor, a complicated optical evaluation and the use of often not CMOS compatible materials.

So-called pyroelectric radiation sensors have pyroelectric materials with electrodes arranged on opposite surfaces. As pyroelectric materials serve, for example, thin platelets which form the individual pixels and which are formed of lithium tantalate or preferably lead zirconate titanate (PZT). The sensor elements form capacitors with a small capacitance. At a temperature change of the pyroelectric material, so also as a result of the absorption of electromagnetic radiation, the polarization of the material changes. The resulting redistribution of equalizing charges on the surface of the material causes a current to flow in an upstream sensor circuit.

The change in the polarization of the pyroelectric layer is measured by the change in temperature upon absorption of electromagnetic radiation, preferably in the wavelength range between 7μηη and 15μηη, in which objects around room temperature emit most of their thermal radiation. The polarization per se is usually measured via a charge amplifier. The amount of polarization change is dependent on the rate of change of the temperature, with the largest possible measurement signal is desirable due to a large rate of change. The disadvantage here is the thermal inertia of the pixels, which results in a long-term drift, since the pixel, which generally has a free surface above the substrate, heats up as the irradiation duration increases. The consequence of this long-term drift is a decreasing polarization change, resulting in disadvantageously a signal attenuation.

DISCLOSURE OF THE INVENTION

The object of the invention is to improve the structure and function of a pyroelectric sensor for detecting electromagnetic radiation, in particular heat radiation. In particular, the sensor should have improved thermal behavior and enable better signal stability.

This object is achieved on the basis of a pyroelectric sensor according to the preamble of claim 1 with the characterizing features. Advantageous developments of the invention are given in the dependent claims.

In order to achieve the object, the method according to the invention provides the following steps: irradiating the pyroelectric layer with an electromagnetic radiation, producing at least indirect contact between the pixel and the substrate material by moving the pixel towards it Substrate material out of a starting position, so that a thermal short circuit between the pixel and the substrate material is prepared, reading a charge transfer in the pyroelectric layer on the bottom electrode and the roof electrode and returning the pixel in the initial position spaced from the substrate material.

Core of the invention is a rapid temperature change in the pyroelectric layer by a thermal short circuit with the substrate material, so that an increase in the charge transfer is achieved, which in turn can lead to a stronger measurement signal. In other words, the pyroelectric layer can be brought to substrate temperature very rapidly, and the rapid temperature change leads to a large change in polarization in the material of the pyroelectric layer and consequently to a large measurement signal. In the starting position, the pixel is substantially levitating over the substrate material and has no significant solid state contacts, so that when exposed to electromagnetic radiation, the pyroelectric layer of the pixel can strongly heat up. Only upon completion of the irradiation can a charge shift by the temperature change be measured by adjusting the polarization change by the temperature change and the stronger the temperature change, the greater the polarization change and the stronger the measurable signal becomes. Thus, according to the invention, the pixel is strongly cooled by the substrate material, wherein the cooling is preferably generated only for at least one readout of the charge transfer in the pyroelectric layer. By subsequently returning the pixel to the starting position again spaced from the substrate material, heating of the pixel can take place again by renewed irradiation of the pyroelectric layer with electromagnetic radiation.

In order to carry out the method according to the invention, the pyroelectric sensor has a first short-circuit electrode arranged at the pixel and a second one

Shorting electrode disposed on the substrate material, and drawing the pixel to the substrate material from the starting position is carried out by applying an electrical voltage between the first short-circuiting electrode and the second short-circuiting electrode. The drawing of the pixel onto the substrate material takes place electrostatically until the pixel reaches the sub- stratmaterial touched and in a way a thermal short circuit is generated. Due to the thermal short circuit, the heat introduced by the electromagnetic radiation into the pixel can be quickly dissipated into the substrate material, which leads to a strong measurement signal due to the resulting charge displacement.

The return of the pixel in the substrate material spaced starting position is effected by switching off the voltage between the first short-circuit electrode and the second short-circuit electrode. The switching off of the voltage between the short-circuiting electrodes takes place when the charge displacement has been measured, wherein the charge displacement is generally also referred to as pyroelectric voltage.

With further advantage, the time of producing the at least indirect contact between the pixel and the substrate material and the time of reading out the charge transfer in the pyroelectric layer coincide at a common time. As soon as the contact is established, and thus the thermal short circuit, the readout of the charge transfer and thus the measurement of the pyroelectric voltage takes place, and as soon as the short-circuiting electrodes are no longer energized, the measurement is stopped.

Another advantage is the possibility that after reading the charge transfer in the pyroelectric layer on the bottom electrode and the roof electrode, a further measurement is made, wherein by a

Difference formation of the measured signals from the successive charge shifts offset and noise components can be eliminated. Consequently, in addition to the described signal increase and the advantage can be used that a so-called "correlated double-sampling" is possible, in which by measurements once with and once without signal, ie before and after the

Establishment of the thermal short circuit, the noise components contained in both measurements can be separated from the signal and eliminated. The method of external shoppering of the signal source, which is partly used in the prior art, for example by a mechanical component which interrupts the beam path periodically, achieves the same advantage, but can often be done not or only with significant additional costs and other disadvantages such as noise development realize.

The invention is further directed to a pyroelectric sensor for measuring electromagnetic radiation, in particular heat radiation, wherein the sensor has a pixel array with a plurality of pixels, which are movably received on a substrate material, and wherein the pixels have a pyroelectric layer, the at least is formed in sections between a bottom electrode and a roof electrode of the pyroelectric sensor, and according to the invention, the pyroelectric sensor has a first shorting electrode arranged on the pixel and a second shorting electrode in arrangement on the substrate material, wherein when applying a voltage to the shorting electrodes at least indirectly between the pixel and the substrate material, a contact can be produced. With the pyroelectric sensor, the method described above is practicable and the contact between the pixel and the substrate material leads to a thermal short circuit, so that the pixel has a high temperature change rate, which high temperature change rate leads to a large polarization change. A correspondingly large measurement signal can consequently be measured by a corresponding readout unit.

The pyroelectric sensor according to the present invention has a plurality of pixels, wherein the substrate material for the plurality of pixels can be formed uniformly and in one piece.

For example, the first short-circuiting electrode is formed on the pixel by the bottom electrode of the pixel. Consequently, only a single electrical connection is used to form both the bottom electrode and the short-circuit electrode, with the applied electrical signals and voltages between the bottom electrode and the roof electrode and between the short-circuit electrodes being galvanically separated from one another.

It is also advantageously provided that a cavity is formed in the substrate material below the pixel, wherein the second short-circuiting electrode is formed below the pixel on the bottom of the cavity. The cavity can either be formed by an etched recess in the substrate material, or there are posts arranged laterally to the pixel, which are constructed on the surface of the substrate material. Consequently, a cavity or cavern arises between the posts, and on the bottom of the cavity or cavern, the second shorting electrode is located immediately below the pixel. The second shorting electrode may have a non-stick coating to prevent the pixel from adhering to the bottom of the cavity.

According to a further advantageous embodiment of the pyroelectric sensor, the pixels are accommodated by means of a pair of mechanical support structures, wherein the bottom electrode with a first mechanical support structure and the roof electrode with a second, in particular opposite mechanical support structure are at least structurally identical and / or arranged on the support structures. The support structures advantageously have a meandering shape, wherein the bottom electrode and / or the roof electrode form a respective thin-film conductor on the meander-shaped support structures. The meandering shape of the support structures makes it possible to make them very long, so that the pixel is essentially thermally decoupled from the substrate material.

PREFERRED EMBODIMENT OF THE INVENTION

Further, measures improving the invention will be described in more detail below together with the description of preferred embodiments of the invention with reference to FIGS. It shows:

FIG. 1 shows a plan view of a first exemplary embodiment of a pyroelectric sensor with the features of the invention,

FIG. 2 shows a cross-sectional view through the pyroelectric sensor according to FIG. 1,

Figure 3 shows a second embodiment of a pyroelectric sensor with the features of the invention and FIG. 4 shows a cross-sectional view through the exemplary embodiment of the pyroelectric sensor according to FIG. 3.

Figure 1 shows a plan view of a pyroelectric sensor 1 in a schematic representation. Laterally there are parts of the substrate material 11, which are designed in the form of posts 19. The pyroelectric sensor 1 has as an essential component a pixel 10, wherein an array of many pixels 10 is formed to form a pyroelectric sensor 1, and only a single pixel 10 is shown for the present illustration. The pixel 10 is movably received on support structures 18, which are executed meander-shaped. With the support structures 18, the pixel 10 is held between the posts 19, and the support structures 18 are formed opposite to the pixel 10 and hang the pixel 10 between the two posts 19 of the substrate material 11 shown.

The pixel 10 has, as an essential structure, a pyroelectric layer 12, and the pyroelectric layer 12 is contacted via two electrodes, the plan view showing the roof electrode 14. This is designed rectangular and leaves a particular inner part free, so that an electromagnetic radiation can directly irradiate the pyroelectric layer 12 without this would be covered on the top side of the roof electrode 14.

A bottom electrode 13 is electrically isolated from the roof electrode 14 with an insulation 21 and contacts the pyroelectric layer 12 from the underside. Since the bottom electrode 13 engages under the pyroelectric layer 12, only a strip-like side view of the bottom electrode 13 can be seen. The insulation 21 extends between the roof electrode 14 and the bottom electrode 13.

The bottom electrode 13 is formed in a manner not shown in detail structurally with the left support structure 18, and the roof electrode 14 is structurally formed with the right support structure 18.

Figure 2 shows a cross-sectional view of the pyroelectric sensor 1 according to the embodiment in Figure 1. The cross-sectional view illustrates the Forming the substrate material 11 with laterally formed posts 19, between which the pixel 10 is suspended via the support structures 18. On top of the support structures 18 are the bottom electrode 13 on the left side and the roof electrode 14 on the right side, and the bottom electrode 13 engages under the pyroelectric layer 12 and the roof electrode 14 passes over the pyroelectric layer 12 with a window-like opening as in FIG Top view shown in Figure 1. Furthermore, the cross-sectional view shows the insulation 21 for electrical insulation between the bottom electrode 13 and the roof electrode 14.

In the substrate material 11, a cavity 17 is formed by the formation of the posts 19, into which the pyroelectric layer 12 extends. The bottom electrode 13 furthermore forms a first short-circuit electrode 15, and on the bottom of the cavity 17 there is a second short-circuit electrode 16, which is coated on the upper side with an anti-adhesion layer 22. If a voltage is applied between the first short-circuit electrode 15, formed by the bottom electrode 13, and the second short-circuit electrode 16, contacted via a feed line 20, then the pixel 10 is electrostatically attracted to the bottom of the cavity 17 and thus to the substrate material 11. The result is a very rapid cooling of previously heated with an electromagnetic radiation pixel 10. This allows a high temperature change rate can be achieved, whereby a large pyroelectric signal on the bottom electrode 13 and the roof electrode 14 can be tapped. Figure 2 shows a slightly modified embodiment of the pyroelectric

Sensor 1 with a pixel 10, which is suspended over the support electrodes 18 between two posts 19 of a substrate material 11. On the top of the pyroelectric layer 12, the roof electrode 14 is shown, which is insulated with an insulation 21 against a bottom electrode, which is not shown in detail.

The cross-sectional view in Figure 4 shows the embodiment of Figure 3, and the pyroelectric layer 12 is in contact between the bottom-side bottom electrode 13 and the top-side roof electrode 14. The bottom electrode 13 is constructed on the right side integral with the support structure 18, and the roof electrode 14 is structurally on the left with the support structure 19 executed. Thus, the pixel 10 is movably received between the two posts 19 of the substrate material 11. The underside of the pixel 10 extends into the cavity 17 and has a first short-circuiting electrode 15, which is embodied separately from the bottom electrode 13. On the bottom of the cavity 17 is the second shorting electrode 16 with an anti-adhesion layer 22 applied thereto.

The voltage supply of the short-circuiting electrodes 15 and 16 can be effected galvanically separated from the voltage measurement between the bottom electrode 13 and the roof electrode 14. The contacting of the second short-circuit electrode 16 via the supply line 20, the contacting of the first short-circuit electrode 15 is not shown in detail.

The invention is not limited in its execution to the above-mentioned preferred embodiment. Rather, a number of variants is conceivable, which makes use of the illustrated solution even with fundamentally different types of use. Any features and / or advantages resulting from the claims, the description or the drawings, including constructive details, spatial arrangements and method steps, can be essential to the invention, both individually and in the most diverse combinations.

Claims

claims

1. A method for measuring electromagnetic radiation, in particular thermal radiation, with a pyroelectric sensor (1), wherein the pyroelectric sensor (1) has a pixel array with a plurality of pixels (10) which are movably received over a substrate material (11) and wherein the pixels (10) comprise a pyroelectric layer (12) formed at least in sections between a bottom electrode (13) and a roof electrode (14) of the pyroelectric sensor (1), the method comprising at least the following steps:

Irradiating the pyroelectric layer (12) with electromagnetic radiation,

Producing at least indirect contact between the pixel (10) and the substrate material (11) by advancing the pixel (10) against the substrate material (11) from a starting position so that a thermal short circuit between the pixel (10) and the substrate material (11 ) will be produced,

Reading a charge transfer in the pyroelectric layer (12) via the bottom electrode (13) and the roof electrode (14) and

Returning the pixel (10) into the starting position spaced apart from the substrate material (11).

2. The method according to claim 1, characterized in that the pyroelectric sensor (1) has a first shorting electrode (15) arranged on the pixel (10) and a second shorting electrode (16) in arrangement on the substrate material (11), wherein the moving of the pixel (10) to the substrate material (11) from an initial position by applying an electric voltage between the first short-circuiting electrode (15) and the second short-circuiting electrode (16).

3. The method according to claim 1 or 2, characterized in that the return of the pixel (10) to the substrate material (11) spaced starting position by switching off the voltage between the first short circuit conclusion electrode (15) and the second short-circuit electrode (16) is executed.

4. The method according to any one of claims 1 to 3, characterized in that the time of producing the at least indirect contact between the pixel (10) and the substrate material (11) and the time of reading out the charge transfer in the pyroelectric layer (12) coincide at a common time.

5. The method according to any one of the preceding claims, characterized in that after reading the charge transfer in the pyroelectric layer (12) via the bottom electrode (13) and the roof electrode (14) a further measurement is made, wherein by a difference of measured signals The charge shifts offset and noise components are eliminated.

6. pyroelectric sensor (1) for measuring electromagnetic radiation, in particular heat radiation, wherein the sensor (1) has a pixel array with a plurality of pixels (10), which are movably received on a substrate material (11), and wherein the pixels (10 ) have a pyroelectric layer (12) which is at least partially formed between a bottom electrode (13) and a roof electrode (14) of the pyroelectric sensor (1),

characterized in that the pyroelectric sensor (1) has a first shorting electrode (15) arranged on the pixel (10) and a second shorting electrode (16) disposed on the substrate material (11), wherein upon application of a voltage to the shorting electrodes (15 , 16) a contact can be produced at least indirectly between the pixel (10) and the substrate material (11).

7. pyroelectric sensor (1) according to claim 6, characterized in that the first short-circuit electrode (15) in arrangement on the pixel (10) through the bottom electrode (13) of the pixel (10) is formed.

8. pyroelectric sensor (1) according to claim 6 or 7, characterized in that below the pixel (10) a cavity (17) in the substrate material (11) is formed, wherein the second shorting electrode (16) below the pixel (10) the bottom of the cavity (17) is formed.

9. pyroelectric sensor (1) according to any one of claims 6 to 8, characterized in that the pixels (10) by means of a pair of mechanical support structures (18) are accommodated, wherein the bottom electrode (13) with a first mechanical support structure (18). and the roof electrode (14) having a second, in particular opposite, mechanical support structure (18) is designed to be at least structurally uniform and / or arranged on the support structures (18).

10. pyroelectric sensor (1) according to claim 9, characterized in that the support structures (18) have a meandering shape, wherein the bottom electrode (13) and / or the roof electrode (14) form a respective thin-film conductor on the meandering support structures (18).