FR2788885A1 - Thermal detection device for electromagnetic radiation, especially near IR, has independent mechanical connections for interconnecting suspended layers of adjacent micro-bridge detectors - Google Patents

Abstract

Electromagnetic radiation thermal detection device has independent mechanical connections (15, 15') for interconnecting suspended layers of adjacent micro-bridge detectors. A thermal detection device for electromagnetic radiation has mechanical supports between micro-bridge detectors and a signal processing circuit, the suspended layers of the micro-bridges of adjacent detectors (16, 17, 18) being interconnected by supplementary mechanical connections (15, 15') distinct from the mechanical supports. An Independent claim is also included for production of the above device.

Description

THERMAL RADIATION DETECTION DEVICE

  ELECTROMAGNETICS AND METHOD FOR MANUFACTURING

THIS ONE

DESCRIPTION

  Technical Field The present invention relates to a device for thermal detection of electromagnetic radiation,

  and a method of manufacturing the same.

  STATE OF THE PRIOR ART An electromagnetic radiation detector based on the principle of thermal detection, as shown diagrammatically in FIG. 1, is generally made up of different sub-assemblies which carry out the four essential functions necessary for the detection of radiation, namely: - an absorption function: The absorption function converts the energy of the incident electromagnetic wave, which is characteristic of the temperature and emissivity of the scene observed, into a heating of the structure of detection. The parameters which characterize this function are: * On the one hand the relative absorption (Ar) which defines the ratio of the luminance of the incident radiation to the luminance actually absorbed by the absorbing structure. A quarter-wave resonant optical cavity provides relative absorption

  close to the ideal value of 100%.

  * On the other hand, the filling factor (Fr) which is the ratio of the useful surface actually participating in the heating of the detector to the - E - total surface of the latter. We thus obtain

  filling factors of the order of 50%.

  Optimizing the absorption function therefore essentially consists in maximizing these parameters Fr and Ar. - a thermometer function: The thermometer is an element one of whose physical characteristics is sensitive to temperature. It can be the electrical resistivity of the material in the case of resistive bolometers, the conductivity of semiconductor devices, the residual polarization in the case of a pyroelectric detector, the dielectric constant in the case of a ferroelectric detector, etc. .. The essential quality factor that characterizes the function of a thermometer is the relative variation of the physical quantity observed with temperature. For a resistive bolometer with resistance R this factor of

  quality is expressed by dR / R.dT, otherwise noted TCR.

  Optimizing the thermometer consists of maximizing this parameter. - a thermal insulation function: The thermometer is thermally isolated from its environment, for example by placing the thermometer on a membrane suspended above a substrate, according to an architecture called "micro-bridge", which is thermally isolated from on the one hand by integrating the detector into an environment under reduced gas pressure and on the other hand by inserting a specific thermal insulation device between the micro-bridge supporting the thermometer and the downstream signal processing circuit. The characteristic thermal parameters are on the one hand the thermal impedance Rth which must be maximized in order to improve the sensitivity of the detector and on the other hand, the heat capacity Cth which translates the thermal inertia of the thermometer which must be minimized in order to reduce the response time of the detector to a variation in the incident flux. The response time which is proportional to the product Rth x Cth is typically a few

  milliseconds to a few tens of milliseconds.

  In order to achieve a sensitive and rapid detector at the same time, it is sought to maximize the efficiency of the thermal insulation and to minimize the volume of the thermometer. This optimization involves the

  realization of structures in thin layers.

  - a signal processing function: The signal processing function consists in translating the electrical signal delivered by the thermometer into a video signal which can be used by a camera. This function is carried out: * Either by hybridization of the detection circuit on the treatment circuit: this first solution, which requires treating each component individually, is incompatible with a process o the technological manufacturing operations are carried out simultaneously on a large number of components assembled flat on a substrate. This first solution therefore poses the problem of manufacturing cost

Student.

  * Or by assembling the detector on a micro-bridge suspended above a pre-existing processing circuit. The component produced is then called "monolithic". This second solution which makes it possible to get rid of the problem of the manufacturing cost imposes severe constraints on the technological processes which realize the detection structure: in particular the thermal budget must be limited in order not to degrade the performances

  electrics of the treatment circuit.

  In addition to these various functions, it is also necessary: * On the one hand to carry out the mechanical maintenance between the detector and the processing circuit. * On the other hand ensure the transmission of the electrical signal from the thermometer to the circuit

treatment.

  Figures 2 and 3 schematically represent the layout of the different functions necessary for detection. Figure 2 refers to an architecture where the detector is assembled above the processing circuit, while Figure 3 represents

  a configuration where these two elements are juxtaposed.

  In these two figures are shown: a zone 10 which constitutes the thermometer and corresponds to the active zone of the detector which actually collects the incident photons; - Zones 11 which constitute the mechanical holding devices and electrical interconnection between the detector and the processing circuit; - the zones 12 which constitute the thermal insulation devices of the detector; - a zone 13 which represents the circuit of

signal processing.

  In Figure 2, area 13 is not

  shown because it is located under the detector.

  The devices 11, 12 and 13 do not participate in the detection; in order to maximize the filling factor, it is therefore sought to limit the area necessary for their realization, by: - limiting their number to a strict minimum, for example two; - limiting their size, reducing the length of the thermal insulation devices, and therefore their section and their thickness in order to maintain a

sufficient thermal insulation.

  - favoring the architecture where the detector is assembled on the processing circuit according to a

monolithic architecture.

  European patent application EP-0 354 369 describes, therefore, an uncooled monolithic infrared detector network of bolometers manufactured on a silicon substrate. The bolometers include a stack of silicon oxide, TiN (titanium nitride), a- Si: H (hydrogenated amorphous silicon), TiN, silicon oxide. The titanium nitride forms the infrared absorber and the resistance contacts, and the amorphous silicon the resistance with a high temperature coefficient of resistivity. The resistor is suspended above the silicon substrate by metallic interconnections and the processing circuit

  associated is formed in the silicon substrate au-

below the resistance.

  To minimize the mechanical deformations of the fine structures used, a first solution consists in compensating for the stresses which develop in a thin layer by the arrangement of a layer.

  additional in contact with this layer.

  A second solution consists in reducing the amplitude of the intrinsic stresses of the materials used by resorting to thermal treatments

  at high temperatures to relieve stress.

  However, this solution results in thermally constraining the electronic processing circuit placed under the detector and degrading the functionality of said circuit. We will now consider several examples

  according to the prior art.

  FIG. 4 represents a perspective view of a unitary detector characterized by thermal insulation devices 12 of intermediate length. The structures most often produced, illustrated in FIGS. 5, 6 and 7, represent a plan view of three neighboring detectors 16, 17 and 18 forming part of a generally more complex structure, linear strip or two-way stamping

detector dimensions.

  In the embodiment illustrated in FIG. 5, the thermal insulation is maximized by means of very long thermal insulation devices 12 associated with mechanical holding devices and electrical interconnection 11. This embodiment has the following drawbacks: an active area 10 reduced due to the size of the insulation devices, hence a low filling factor; - a tendency for part 12 to bend due to its length, which requires a membrane

  thicker to ensure mechanical rigidity.

  In the embodiment illustrated in FIG. 6, the filling factor is maximized by limiting the area devoted to the thermal insulation devices 12; mechanical deformations are limited and a fine structure can be used. However, this embodiment has reduced thermal insulation and, consequently, limited detection sensitivity. In the embodiment illustrated in FIG. 7, four physical links are introduced between the detector and the processing circuit, said links being made up of thermal insulation devices 12 associated with mechanical holding devices and electrical interconnection 11. This embodiment provides good mechanical stability of the structure and thin film detectors. However, this embodiment has the following drawbacks: - an active area 10 reduced due to the number and size of the isolation devices 12 and of the mechanical holding and electrical interconnection devices 11; the filling factor of this type of detector is therefore low; lower thermal insulation because thermal leaks can be divided into eight branches instead of two, resulting in a loss of sensitivity

by a factor of 4.

  The object of the invention is to propose a device for thermal detection of electromagnetic radiation comprising micro-bridge thermal detectors using suspended active layers.

  the thinnest and flattest possible.

  DESCRIPTION OF THE INVENTION The present invention relates to a device for the thermal detection of electromagnetic radiation, for example infrared or millimeter radiation, comprising at least two microbridge detectors, in which the suspended layers of the micro-bridges of two neighboring detectors are interconnected by a connection

mechanical.

  Advantageously, each mechanical connection is an extension of at least one of the layers

suspended micro-bridges.

  Advantageously, each mechanical connection comprises a material with low heat conductivity. Advantageously the (or) connection (s) mechanical (s) is (are) in alignment with two mechanical holding devices, each belonging to

one of two neighboring detectors.

  Advantageously, the device of the invention can be connected to one or more neighboring devices by forming a repetitive configuration of said detector according to a linear or matrix architecture suitable for producing images of sources of electromagnetic waves. The invention relates more particularly to the field of infrared detectors based on the principle of thermal detection as opposed to quantum detection, and advantageously operating at

ambient temperature.

  The invention also relates to a method of manufacturing such a device starting from a treatment circuit showing on the surface metallic bonding pads, passive by an insulating layer in which openings are provided at the pads. This process comprises the following stages: - a reflector is produced on the surface of the treatment circuit by deposition of a metal layer and definition by photolithography; - An optical cavity is produced by deposition and annealing of a sacrificial layer which is then removed; - At least two layers constituting the micro-bridge are deposited, namely * a layer of temperature-sensitive material, * a conductive layer constituting the electrodes of the detector; - mechanical holding devices and electrical interconnection are made * by etching at the right of the connection pads, the sacrificial layer, the layer of temperature-sensitive material and the conductive layer, * by depositing and etching at least one metal layer which ensures electrical and mechanical continuity between the connection pads and the electrodes of the micro-bridge; - The electrodes of the detector are defined by etching the conductive layer; - the layer of temperature-sensitive material, the conductive layer and the optional layers necessary for making the micro-bridge are simultaneously etched, using a mask to spare a

area between the detectors.

  Advantageously, the following characteristics can be obtained. The layer of temperature sensitive material is a layer of amorphous silicon. The conductive layer constituting the electrodes of the detector is a layer of titanium nitride. The metal layer, which ensures electrical continuity between the electrical studs and the micro-bridge electrodes, is a layer of aluminum. The metallic layer, constituting the electrodes of the detector is removed, in the areas occupied by the mechanical connections. After the step of defining the electrodes of the detector by etching the conductive layer, a final

* 1 -_ _

  layer, which can be a layer of silicon oxide,

  silicon nitride, or amorphous silicon.

  In a first alternative embodiment, the connection devices are thinned by partial etching of the latter. Advantageously, the conductive layer and the layer can be eliminated.

  optional at the connection level.

  In a second alternative embodiment, on micro-bridges completely isolated from each other, a connection element produced in a

  material other than those already present in the micro-

  bridge and having a low heat conductivity: for example silicon nitride or a

polymeric material.

  The invention makes it possible to obtain the following advantageous results: * The efficiency of the absorption of the incident wave is optimized, thanks to a better geometric conformation of the optical cavity which is

  a quarter-wave resonant cavity.

  * The realization of very thin structures, typically 100 nanometers, or even less is made possible, and no longer of the order of 500 nanometers, as in the devices of the prior art. The implementation of a thin-layer micro-bridge also makes it possible to reduce the thermal inertia of the detector, and consequently leads to the production of faster detectors with respect to

modulations of the incident flow.

  * By favoring the active area which effectively participates in the collection of incident photons, the fill factor is increased. The sensitivity of the detector is therefore increased. Typically the invention makes it possible to obtain a filling factor of the order of 80%, which is much greater than the factor of

  filling of order of 50% of the prior art.

  e The mechanical deformations induced by the intrinsic stresses of the layers constituting the micro-bridge are compensated for by the mechanical connections. The components produced therefore do not require stress relieving heat treatments. The signal processing circuit can thus advantageously be integrated into the detection circuit according to a monolithic structure, which is preferable to a hybrid structure, in terms of

performance and costs.

Brief description of the drawings

  Figure 1 illustrates the block diagram of a conventional electromagnetic radiation thermal detector. Figures 2 and 3 schematically represent the implementation of the different functions necessary for

detection.

  Figures 4, 5, 6 and 7 illustrate several

  conventional detector structures.

  FIG. 8 illustrates a first embodiment of the detection device according to

the invention.

  FIG. 9 illustrates a second embodiment of the detection device according to the invention. FIG. 10 shows the template of the filter suitable for processing a signal from a central detector having two connection elements to the

neighboring detectors.

  FIGS. 11A and 11B illustrate two sectional views of a structure produced according to a preferred mode of

  the invention in the field of infrared detection.

  Figure 12 illustrates the drawing of a mask which

  cuts a micro-bridge according to the invention.

  Detailed description of embodiments

  In the following description, the elements

  analogous to those of the devices of the prior art

  described above retain the same references.

  The present invention relates to a device for the thermal detection of electromagnetic radiation comprising at least two micro-bridge detectors, in which the "suspended" layers of the micro-bridges are connected together by a mechanical connection. These suspended layers are the layers of the micro-bridge which

  are physically isolated from the substrate and maintained

  above the substrate by mechanical holding devices. This device, shown in FIG. 8, comprises the following elements: two mechanical holding devices and electrical interconnection 11 per detector; - two thermal insulation devices 12 per detector; - an active area sensitive to radiation 10 by detector; - two mechanical connections 15, 15 'which mechanically connect the central detector 16 to neighboring detectors 17 and 18, and which prevents weakening of the most micro-bridge areas

  away from mechanical holding devices 11.

  Each mechanical connection 15, 15 ′ can be an extension of at least one of the suspended layers of the micro-bridges. It can be made up of a

  material with low heat conductivity.

  The device of the invention has mechanical stability reinforced by specific holding devices, which ensure mechanical continuity between each detector and its closest neighbors. The realization of a repetitive configuration of the detector of the invention according to a linear or matrix architecture leads to an assembly of detectors which will be described as related, the holding of which

mechanics is improved.

  The IMT thermal intermodulation which results in an electrical intermodulation between neighboring detectors is perfectly defined by the respective geometric dimensions of the thermal insulation devices 12 and of the mechanical connections 15 and 15 ′ and therefore can be corrected. At the first order, we have the following relationships: IMT = dT / dTv = Rth / (Rth + 2.Rcx) Rth = L1 / (Xl.W1.El) Rcx = L2 / (2.W2.E2) with: * dT the heating of a detector, induced via the mechanical connections, by the heating dTv of the neighboring detector receiving the infrared flux; * Rth the thermal impedance of the thermal insulation devices 12; * Rcx the thermal impedance of the mechanical connections 15 and 15 '; kl, L1, W1, E1 being respectively the heat conductivity, the length, the width and the thickness of the thermal insulation devices 12 and k2, L2, W2, E2 representing the same parameters relating to the mechanical connections 15, 15 'In this particular case where the devices 12 and 15 ′ have the same section and a

  identical heat conductivity, the inter-

  IMT modulation between detectors is expressed by:

IMT = L1 / (L1 + 2.L2)

  The intermodulation between detectors can therefore be limited and adjusted according to the intended application, thanks to a suitable drawing of the devices 12, 15 and 15 '. Typically values of the order of 20%, which make it possible to produce a good quality infrared retina, can be obtained for connections 15, 'having a double length of the isolation arms.

  12, as shown in Figure 9.

  One can also completely get rid of the intermodulation introduced by the connections by carrying out an adapted mathematical processing of the signal coming from the detectors, by deconvoluting (reverse filtering) the raw signal marred by intermodulation by a filter whose template is defined by the intermodulation rate. FIG. 10 thus represents the template of a filter adapted to the processing of a signal coming from a central detector 16 having two elements of connections to the neighboring detectors 17 and 18 and

  characterized by an intermodulation rate of 10%.

  We will now describe several modes of

  realization of the device of the invention.

  FIGS. 11A and 11B show two sectional views of a structure produced according to a preferred embodiment of the invention, showing two neighboring detectors 16 and 17. The first section (FIG. 11A) is produced outside the connection devices 15 and 15 ', then

  that the second (Figure 11B) crosses them.

  The method of manufacturing such a device starts from a processing circuit 19 already completed, obtained

  according to known techniques for example micro-

  electronic on silicon, showing on the surface metallic connection pads 20 which make it possible to make the electrical connections between

  detectors and inputs to the processing circuit.

  These connecting pads 20 are usually passive by an insulating layer 21 in which openings have

  have been fitted out at the studs.

  A metal layer 22, for example of aluminum, is advantageously deposited and defined by photolithography in order to produce an infrared reflector on the surface of the processing circuit 19. The role of this reflector is to optimize the absorption of the infrared wave by improving the efficiency of the quarter-wave resonant cavity constituted by the reflector 22, the micro-bridge 29 and the space between these

two elements.

  A sacrificial layer 23, composed for example of polyimide, is then extended and optionally annealed. This layer on which the micro-bridge is assembled and which is ultimately removed, makes it possible to produce said cavity. The thickness of this layer is generally 2.5 micrometers, which makes it possible to produce a sensitive detector in a range

  wavelength on the order of 10 micrometers.

  The layers constituting the micro-bridge, which are then deposited on the sacrificial layer 23, are at least two in number: - a layer 24 of temperature-sensitive material which can be amorphous silicon deposited according to a conventional process; a conductive layer 25 constituting the electrodes of the detector, which may be nitride of

  titanium deposited by reactive spraying.

  The mechanical holding and electrical interconnection devices, the embodiment of which will be described below, are those of a micro-bridge in the infrared domain. The steps for obtaining them are specific to them, independent of the previous steps described and can be replaced by the steps for obtaining other holding and interconnection devices. These mechanical holding devices and electrical interconnection are thus obtained by carrying out: an etching, according to photolithography methods, of the layers 23, 24, 25 in line with the connection pads 20; - Then, the deposition of one or more metallic layers 26 which ensure electrical and mechanical continuity between the connection pads and the electrodes of the micro-bridge. This metallic layer consists, for example, of aluminum. This layer 26 is defined and etched according to conventional methods, so as to limit the size of these interconnection devices to the only surface necessary for good resumption of contact with the electrode 25 of the detector. The electrodes of the device of the invention are then defined by etching the metal layer 25 according to a configuration adapted to the characteristics

  that we want to give to the detector.

  This layer 25 is advantageously removed from the zones which will subsequently be occupied by the connection members, so as to avoid electrical short circuits between detectors and to improve the impedance

thermal connections.

  One can also deposit on the micro-bridge 29 a last layer 28 which makes it possible to obtain a symmetrical structure less sensitive to the internal stresses which develop in the layers, by compensating for phenomena of the "bimetal" type. This layer 28 can be either an electrically active material, possibly of the same nature as the temperature-sensitive material 24, or an electrically neutral material which can be a material with low heat conductivity because it can increase the thermal leaks of the micro-bridge. It is therefore preferable to use silicon oxide,

  silicon nitride or amorphous silicon.

  A last photolithographic level makes it possible to define the perimeter of the detectors by simultaneous etching of the layers 24, 25, 28, which makes it possible: to isolate the detectors from each other; - Define the thermal insulation devices 12 cut in the micro-bridge 29 proper, so as to achieve between the mechanical holding device and electrical interconnection on the one hand and the detector on the other hand, a reduced section, long length and good mechanical strength. Connections between detectors can also be made during this last step. By using a mask with an adequate design, the etching of the layers 24, 25, 28 spares a particular area, of limited extent and located between the detectors, the spared material constituting the connection devices. The spared area has a small section, typically 0.5 to 3 micrometers wide

  for a thickness equal to the thickness of the micro-bridge.

  The geometric ratio of the connection to the total perimeter of the detector is then very limited, which makes it possible to produce detectors having a low

thermal intermodulation.

  We will now consider successively several variants of the device of the invention intended to limit the thermal intermodulation between detectors, while ensuring satisfactory mechanical strength. A first variant of the invention consists in thinning the connection devices by partially etching them. One can either completely engrave one of the layers of the connection elements, or substantially thin one of its components by controlling the etching time. For example, the metal layer 25 and the optional layer 28 can be eliminated at the connections, without limiting the mechanical strength of the assembly.

  This local etching process calls for the use of a particular mask and the techniques of

usual photolithography.

  A second variant consists in attaching to micro-bridges completely isolated from each other, a connection element made of a material possibly other than those already present in the micro-bridge and chosen for its favorable thermal characteristics, for example nitride of silicon or polymeric materials that have low heat conductivity. Polymers of the PVDF type are particularly favorable since they have a heat conductivity lower by an order of magnitude than the heat conductivity of silicon oxide. The usual deposition techniques, in particular PECVD, LPCVD deposition, sputtering, spreading of solution containing a liquid precursor, etc.

usable.

  the invention can also be applied to connection devices of geometric shape other than rectangular. A design that maximizes the length is favorable since it limits the intermodulation between detectors. By way of example, FIG. 12 shows the drawing of a mask which cuts out the micro-bridge

  according to the concept of the invention and which maximizes the length of the connections.

- I - - - _

Claims (16)

  1. A device for the thermal detection of electromagnetic radiation comprising at least two micro-bridge detectors, characterized in that the suspended layers of the micro-bridges of two neighboring detectors (16, 17, 18) are connected between   them by mechanical connections (15, 15 ').   2. Device according to claim 1, wherein each mechanical connection (15, 15 ') is an extension of at least one of the suspended layers micro-bridges.   3. Device according to claim 1, wherein each mechanical connection (15, 15 ') comprises a   material with low heat conductivity.   4. Device according to claim 1, wherein the (or) connection (s) mechanical (s) (15, 15 ') is (are) in alignment with two mechanical holding devices (11), each belonging to one of two neighboring detectors.   5. Device according to any one of   previous claims, wherein said   device forms a repetitive configuration of detectors according to a linear architecture or matrix.   6. Method of manufacturing a device according to   any one of the preceding claims,   characterized in that, starting from a treatment circuit (19) showing the metal bonding pads (20) on the surface, it comprises the following steps: - a reflector (22) is produced on the surface of the treatment circuit by deposition a metal layer and definition by photolithography; - An optical cavity is produced by depositing a sacrificial layer (23), which is then removed; - At least two layers constituting the micro-bridge are deposited, namely a layer of temperature-sensitive material (24), * a conductive layer (25) constituting the electrodes of the detector; - Mechanical holding devices and electrical interconnection * are produced by etching the connection pads, the sacrificial layer (23), the layer of temperature-sensitive material (24) and the conductive layer at the right (25), * by depositing and etching at least one metal layer (26) which ensures electrical and mechanical continuity between the connection pads (20) and the electrodes of the micro-bridge (25); - The electrodes of the detector are defined by etching the conductive layer (25); - The layer of temperature-sensitive material (24), the conductive layer (25) and optional layers (28) are simultaneously etched, using a mask to spare an area between the detectors. 7. The method of claim 6, wherein the layer of temperature sensitive material   (24) is a layer of amorphous silicon.   8. The method of claim 6, wherein the conductive layer (25) constituting the electrodes of the detector is a layer of titanium nitride. 9. The method of claim 6, in which an aluminum layer (26) is deposited which ensures electrical continuity between the pads   electrical (20) and the micro-bridge electrodes (25).   10. The method of claim 6, in which the metal layer (25) constituting the electrodes of the detector is removed in the occupied zones.   by mechanical connections (15, 15 ').   11. The method of claim 6, wherein, after the step of defining the electrodes of the detector by etching the conductive layer (25), deposits a last layer (28).   12. The method of claim 11, wherein this last layer (28) is a layer of silicon oxide, silicon nitride, or amorphous silicon.   13. The method of claim 6, wherein the mechanical connections are thinned (15, 15 ')   thanks to a partial engraving of these.   14. The method of claim 13, wherein the conductive layer (25) and the   last layer (28) at the connections.   15. The method of claim 6, wherein there is reported, on micro-bridges completely isolated from each other, a connecting element made of a material which has a low heat conductivity.   16. The method of claim 15, wherein the material with low heat conductivity   is silicon nitride or a polymeric material.