WO1992004737A1 - Infrared detector with a thin substrate and method for producing same - Google Patents

Infrared detector with a thin substrate and method for producing same Download PDF

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Publication number
WO1992004737A1
WO1992004737A1 PCT/FR1991/000713 FR9100713W WO9204737A1 WO 1992004737 A1 WO1992004737 A1 WO 1992004737A1 FR 9100713 W FR9100713 W FR 9100713W WO 9204737 A1 WO9204737 A1 WO 9204737A1
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substrate
micrometers
detector
rear face
infrared
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PCT/FR1991/000713
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French (fr)
Inventor
Pierre Dautriche
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Thomson Composants Militaires Et Spatiaux
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Publication of WO1992004737A1 publication Critical patent/WO1992004737A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
    • H01L31/10—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors characterised by at least one potential-jump barrier or surface barrier, e.g. phototransistors
    • H01L31/101—Devices sensitive to infrared, visible or ultraviolet radiation
    • H01L31/102—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier or surface barrier
    • H01L31/108—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier or surface barrier the potential barrier being of the Schottky type
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/144—Devices controlled by radiation
    • H01L27/146—Imager structures
    • H01L27/14643—Photodiode arrays; MOS imagers
    • H01L27/14649—Infrared imagers
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/0352—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions
    • H01L31/035272—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their shape or by the shapes, relative sizes or disposition of the semiconductor regions characterised by at least one potential jump barrier or surface barrier
    • H01L31/035281—Shape of the body

Definitions

  • the invention relates to infrared detectors, in particular matrix detectors making it possible to provide an image of an object emitting thermal radiation in the infrared.
  • arrays of schottky diodes with platinum silicide deposited on silicon are used. These diodes can absorb infrared rays of wavelength located in the band from 3 to 5 micrometers and provide an electrical signal depending on the intensity of the radiation received.
  • These matrices have the advantage of being able to be produced by technologies similar to those of silicon integrated circuits and therefore they allow a very high image resolution with a high density of integration.
  • the low quantum yield (number of electrons generated per photon passing through the diode) of these diodes is low and requires implementation precautions if we want to obtain sufficient detection sensitivity.
  • each diode being connected directly to a multiplexer element placed exactly opposite the diode; bonding is difficult for arrays with a large number of points but the sensitivity is better since the useful surface of each detection diode can be larger.
  • the detector is illuminated by the rear face of the substrate, that is to say the one which does not carry the detection diodes.
  • the front face is masked by the substrate carrying the multiplexer.
  • the high refractive index of platinum silicide (index n close to 5 around a wavelength of 4 micrometers) makes direct lighting undesirable without adaptation of index ; 50% of the incident radiation would be lost by simple reflection on the platinum silicide; by illuminating from the rear face, it is easier to introduce infrared radiation without losses: on the one hand the silicon substrate has an index of about 3, and on the other hand one can cover the rear face with a layer of 'silicon oxide (index 1, 5) to further facilitate the introduction of radiation.
  • an infrared detector is proposed, the photosensitive detection elements of which, consisting of Schottky diodes, are integrated in a semiconductor substrate whose rear face is very greatly thinned, to a thickness which is at most a few tens of micrometers.
  • the rugged relief of the front surface of the substrate causes reflections of radiation in directions not normal to the fine substrate plane, and that the radiation reflected several times can strike diodes distant from those on which the radiation should produce an image. And it is believed according to the invention that this thinning makes it possible at least to bring the spurious images closer to the main image, so that a highly luminous point may produce a light spot on the screen around the main lighting point but not several tasks distant from each other.
  • FIG. 1 shows schematically a - matrix infrared detector illuminated by its rear face
  • FIG. 3 to 5 show different stages of embodiment of the invention for a monolithic detector mounted on ceramic.
  • FIG. 1 which very schematically represents a conventional infrared detector (either monolithic or hybrid)
  • a silicon substrate has been designated by 10, on the front face of which are integrated silicon detection ion diodes 12 platinum.
  • silicon oxide 14 covers the rear face of the substrate to facilitate the introduction without reflection of the infrared radiation arriving from the rear face.
  • the platinum silicide diodes 12 are themselves covered by a sort of optical cavity constituted by a layer of silicon oxide 16 then a layer of aluminum 18 forming a mirror to reflect the radiation towards the rear. bright after it has passed through the diode; since the absorption in the diode is hardly more than 0.5%, it is in fact advantageous to pass the radiation twice through the diode by means of this mirror.
  • the thickness of the layer 14 is preferably chosen so that the light radiation reflected by the mirror is approximately in phase with the incident radiation, around the center of the band of wavelengths to be detected.
  • the radiation is reflected perpendicularly towards the rear and emerges from the substrate.
  • the substrate has for example a thickness of 500 micrometers and if a significant fraction of radiation is reflected for example with an incidence of approximately 30 ° compared to normal, a spurious image will be produced at approximately 500 micrometers away from the main image, that is to say about twenty pixels apart for pixel dimensions of the order of 25 micrometers.
  • the thickness of the substrate is reduced to dimensions of a few tens of micrometers, and preferably even up to only ten micrometers, so that the same beams reflected obliquely will create a spurious image. very close to the main image (that is to say that in fact they will slightly attenuate the contrast without really creating a stray image).
  • FIG. 2 schematically represents the invention. As in FIG. 1, the dimensional scales are not respected so that the general structure remains intelligible.
  • the silicon substrate 10 is very greatly thinned from the rear, at least opposite the detection diodes (that is to say not necessarily at the periphery of the substrate).
  • the final thickness is less than about 50 micrometers; it is in principle between about 50 micrometers and about 10 micrometers.
  • the thinning is carried out after integration of the photosensitive elements (diodes with platinum silicide in particular) in a semiconductor substrate whose thickness is initially for example from 250 to 500 micrometers.
  • the integration of photosensitive elements can be carried out as in the prior art (techniques of monolithic integration on silicon).
  • Thinning can only be mechanical. by running-in. In that case. the entire rear face is obviously thinned. We can thus reach a thickness which is for example around 50 micrometers.
  • This mechanical thinning can be supplemented by a chemical attack in an etching bath attacking the silicon, on condition of course of protecting the front face of the substrate. We can then thin the substrate until it reaches a thickness of ten micrometers. The improvement in resolution is then still better but of course the manufacturing involves one more step, this step being quite delicate.
  • the chemical thinning is done mainly opposite the diodes and preferably not at the periphery of the substrate.
  • the assembly will be made on the other substrate before thinning, so that there is no problem of behavior mechanics of the thinned substrate.
  • the technology is generally as follows: the substrate carrying the detection diodes is welded to a subst rat carrying the multiplexer, by means of indium balls which correspond to each diode an element of the multiplexer placed opposite this diode. We therefore proceed with the thinning of the substrate after assembly of the two substrates; if there is chemical attack, it is necessary to protect in particular the connections by indium balls and the second substrate.
  • the detectors are mounted on a ceramic substrate and electrically connected to conductive pads of this substrate.
  • the substrate is then bonded by its front face (carrying the detection diodes) to the ceramic since the rear face must remain free for illumination from the rear.
  • the thinning both mechanical and chemical, is carried out after the detector has been mounted on this ceramic.
  • an anti-reflection adaptation layer 14 on the thinned rear face, to facilitate the penetration of infrared radiation under normal incidence.
  • a layer of silicon oxide with a thickness of approximately 700 nanometers; silicon oxide is chosen for the reason that it has an intermediate index between that of air and that of silicon.
  • FIGS. 3 to 5 show by way of example the production steps in the case of a monolithic matrix detector (that is to say carrying not only the photosensitive elements but also a multiplexing circuitry for collecting and processing the electric charges generated in each photosensitive element).
  • the silicon substrate 10 carrying the diodes and the rest of the circuitry is soldered face against face against a ceramic substrate ("flip chip” mounting), connection pads 20 on the front face of the substrate 10 being bonded with a conductive adhesive on conductive pads 22 (screen-printed or photo-etched) of a ceramic substrate 30.
  • This substrate is mainly intended for cooling the detector but it will also serve to ensure the rigidity of the detector after thinning.
  • Bonding of the chip with inversion can also be carried out by a hybridization technique in which microbeads (of indium) formed on the silicon substrate are welded on metal areas formed on the ceramic substrate. It is moreover this latter technique that will be used if the second substrate is not a ceramic substrate and is rather a silicon-based substrate carrying the multiplexing circuitry associated with the detector.
  • the initial thickness el is for example 250 or 500 micrometers: the final thickness e2 is a few tens of micrometers, preferably about 50 micrometers.
  • a chemical thinning can then complete the running-in to descend to a thickness e3 of the order of ten micrometers (FIG. 5). For this, we start by protecting the edges of the substrate with a layer of resin 26 resistant to the chemical attack agents of silicon, so that the connections between the chip 10 and the ceramic substrate are not damaged.
  • the attack solution is composed of acetic acid, hydrofluoric acid and nitric acid. Nitric acid oxidizes silicon, dissolved hydrofluoric acid, and the concentration of acetic acid allows you to choose the attack speed.
  • the attack will be all the more rapid as the doping of the substrate will be higher. If the solution is richer in nitric acid, the surface condition after attack is more homogeneous. It is therefore advantageous to proceed in several stages, first with a solution rich in fluororyric acid and then a solution rich in nitric acid.
  • the detector is produced on an epitaxial substrate P / P +, that is to say a substrate mainly tdet ype P + carrying a lightly doped epitaxial layer of type P of ten micrometers thick for example, it will be easy to eliminate the entire P + region and to keep only the epitaxial part, taking advantage of the selectivity of etching of hydrofluoric acid as a function of doping.
  • the anti-reflective layer 14 can then be deposited.
  • infrared detectors the photosensitive elements of which are schottky diodes with platinum silicide on silicon. It will however be understood that it would be transposable to infrared detectors whose photosensitive elements are different, given that the problem does not come specially from these diodes but rather from parasitic reflections under non-normal incidence.

Abstract

Infrared detectors, particularly matrix detectors comprising platinum silicide diodes as light-sensitive elements, are described. To prevent secondary images whenever a given point is intensely illuminated, said images appearing as spots away from said point, it is suggested that the width of the semiconductor substrate (10) supporting said light-sensitive elements (12) be reduced, by mechanical lapping and optionally thereafter by chemical etching, to between approximately 10 and 50 micrometers.

Description

DETECTEUR INFRAROUGE A SUBSTRAT AMINCI ET PROCEDE DE FABRICATION  INFRARED DETECTOR WITH THINNED SUBSTRATE AND MANUFACTURING METHOD
L'invention concerne les détecteurs infrarouge , notamment les détecteurs matriciels permettant de fournir une image d'un obiet émettant un rayonnement thermique dans l'infrarouge . The invention relates to infrared detectors, in particular matrix detectors making it possible to provide an image of an object emitting thermal radiation in the infrared.
Pour réaliser de tels détecteurs infrarouge , on utilise notamment des matrices de diodes schottky au siliciure de platine déposé sur silicium . Ces diodes peuvent absorber les rayons infrarouge de longueur d'onde située dans la bande de 3 à 5 micromètres et fournir un signal électrique fonction de l'intensité du rayonnement reçu .  To produce such infrared detectors, arrays of schottky diodes with platinum silicide deposited on silicon are used. These diodes can absorb infrared rays of wavelength located in the band from 3 to 5 micrometers and provide an electrical signal depending on the intensity of the radiation received.
Ces matrices ont l'intérêt de pouvoir être réalisées par des technologies similaires à celles des circuits intégrés silicium et par conséquent elles permettent une très haute résolution d'image avec une grande densité d'intégration .  These matrices have the advantage of being able to be produced by technologies similar to those of silicon integrated circuits and therefore they allow a very high image resolution with a high density of integration.
Mais le faible rendement quantique (nombre d'électrons engendrés par photon traversant la diode) de ces diodes est faible et nécessite des précautions de réalisation si on veut obtenir une sensibilité de détection suffisante .  However, the low quantum yield (number of electrons generated per photon passing through the diode) of these diodes is low and requires implementation precautions if we want to obtain sufficient detection sensitivity.
Les détecteurs matriciels peuvent êt re réalisés  Matrix detectors can be realized
soit en technologie puremen t monolithique , dans laquelle on intègre sur un même substrat d e silicium à la fois une matrice de diodes de détection et un mult iplexeur permettant de transmettre les charges engendrées par l 'illumination de chaque diode ; la fabrication est alors particulièrement avantageuse pour des matrices à grand nombre fie points d'image , mais la sensibilité est plus faible puisque la surface des pixels est limitée par la nécessité de réserver une place pour un élément, du multiplexeur à chaque point d'image ;  either in purely monolithic technology, in which one integrates on the same silicon substrate both a matrix of detection diodes and a mult iplexer making it possible to transmit the charges generated by the illumination of each diode; manufacturing is therefore particularly advantageous for matrices with a large number of image points, but the sensitivity is lower since the surface of the pixels is limited by the need to reserve a place for an element, from the multiplexer to each image point ;
soit en technologie hybride flans laquelle un premier substrat , portant, une matrice de diodes , est collé sur un deuxième substrat portant un multiplexeur , chaque diode étant connectée directement à un élément d e multiplexeur placé exactement en regard de la diode ; le collage est difficile pour des matrices à grand nombre de points mais la sensibilité est meilleure puisque la surface utile de chaque diode de détection peut être plus grande . either in blank blank technology in which a first substrate carrying a matrix of diodes is bonded to a second substrate carrying a multiplexer, each diode being connected directly to a multiplexer element placed exactly opposite the diode; bonding is difficult for arrays with a large number of points but the sensitivity is better since the useful surface of each detection diode can be larger.
En principe le détecteur est illuminé par la face arrière du substrat, c'est-à-dire celle qui ne porte pas les diodes de détection . Dans le cas de la technologie hybride , la raison en est évidente :la face avant est masquée par le substrat portant le multiplexeur. Dans le cas de la technologie monolithique c'est parce que l'indice de réfraction élevé d u siliciure de platine (indice n voisin de 5 autour d'une longueur d'onde de 4 micromètres) rend peu souhaitable un éclairage direct sans adaptation d'indice ; on perdrait 50% du rayonnement incident , par simple réflexion sur le siliciure de platine ; en éclairant par la face arrière, on peut plus facilement introduire le rayonnement infrarouge sans pertes : d'une part le substrat de silicium a un indice d'environ 3, et d'autre part on peut recouvrir la face arrière d'une couche d'oxyde de silicium (indice 1 , 5) pour faciliter encore l'introduction du rayonnement .  In principle, the detector is illuminated by the rear face of the substrate, that is to say the one which does not carry the detection diodes. In the case of hybrid technology, the reason is obvious: the front face is masked by the substrate carrying the multiplexer. In the case of monolithic technology, this is because the high refractive index of platinum silicide (index n close to 5 around a wavelength of 4 micrometers) makes direct lighting undesirable without adaptation of index ; 50% of the incident radiation would be lost by simple reflection on the platinum silicide; by illuminating from the rear face, it is easier to introduce infrared radiation without losses: on the one hand the silicon substrate has an index of about 3, and on the other hand one can cover the rear face with a layer of 'silicon oxide (index 1, 5) to further facilitate the introduction of radiation.
Dans les matrices réalisées jusqu'à présent , on a remarqué que des images parasites se formaient parfois sur la matrice . Cela a été constaté en particulier lorsque l'image observée comporte des points fortement éclairés II apparaît alors des taches claires parasites autour de l'imago du point fortement éclairé et à une certaine distance de celu i-ci , à des endroits où l'image devrait normalement être sombre . Ces taches sont gênantes puisqu'elles font croire à la présence d'objets émettant fortement un rayonnement infrarouge à des endroits où il n'y a en fait pas de tels objets .  In the matrices produced so far, it has been observed that parasitic images sometimes form on the matrix. This has been observed in particular when the image observed comprises highly illuminated points II parasitic light spots then appear around the imago of the highly illuminated point and at a certain distance from it, at places where the image should normally be dark. These spots are annoying since they make believe in the presence of objects emitting strongly infrared radiation at places where there are in fact no such objects.
On a pensé d'abord que ce phénomène était dû à des diffractions de rayonnement : il ne faut pas oublier qu'on travaille à des longueurs d'onde qui sont de l'ordre de grandeur des dimensions des motifs intégrés dans la surface du substrat portant les diodes de détection . L'invention part de la remarque que ces images parasites seraient plutôt dues en fait à des réflexions parasites des rayonnements infrarouge . We first thought that this phenomenon was due to radiation diffractions: we must not forget that we are working at wavelengths which are of the order of magnitude of the dimensions of the patterns integrated into the surface of the substrate. carrying the detection diodes. The invention starts from the remark that these parasitic images are rather due in fact to parasitic reflections of infrared radiation.
Selon l'invention , on propose un détecteur infrarouge dont les éléments photosensibles de détect ion , constitués par des diodes Schottky, sont intégrés dans un substrat semiconducteur dont la face arrière est très fortement amincie, jusqu'à une épaisseur qui est au maximum de quelques dizaines de micromètres .  According to the invention, an infrared detector is proposed, the photosensitive detection elements of which, consisting of Schottky diodes, are integrated in a semiconductor substrate whose rear face is very greatly thinned, to a thickness which is at most a few tens of micrometers.
On pense en effet que le relief accidenté de la surface avant du substrat provoque des réflexions de rayonnement dans des directions non normales au plan fin substrat , et que le rayonnement réfléchi plusieurs fois peut venir frapper des diodes éloignées de celles sur laquelle le rayonnemen t devrait produire une image . Et on pense selon l'invention que cet amincissement permet au moins de rapprocher les images parasites de l'image principale, de sorte qu'un point fortement lumineux produira peut être une tache lumineuse sur l'écran autour du point d'éclairement principal mais pas plusieurs tâches éloignées les unes des autres .  It is in fact believed that the rugged relief of the front surface of the substrate causes reflections of radiation in directions not normal to the fine substrate plane, and that the radiation reflected several times can strike diodes distant from those on which the radiation should produce an image. And it is believed according to the invention that this thinning makes it possible at least to bring the spurious images closer to the main image, so that a highly luminous point may produce a light spot on the screen around the main lighting point but not several tasks distant from each other.
D'autres caractéristiques et avantages de l'invention apparaîtront à la lecture de la description détaillée qui suit et qui est faite en référence aux dessins annexés dans lesquels : Other characteristics and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the appended drawings in which:
- la figure 1 représente schémat iquement un - détecteur infrarouge matriciel éclairé par sa face arrière ;  - Figure 1 shows schematically a - matrix infrared detector illuminated by its rear face;
- la figure 2 représente le détecteur modifié selon l'invention ;  - Figure 2 shows the modified detector according to the invention;
- les figures 3 à 5 représentent différentes étapes de réalisation de l'invention pour un détecteur monolithique monté sur céramique .  - Figures 3 to 5 show different stages of embodiment of the invention for a monolithic detector mounted on ceramic.
Sur la figure 1, qui représente t rès schématiquement un détecteur infrarouge (soit monolithique soit hybride) classique , on a désigné par 10 un substrat, de silicium , sur la face avant duquel sont intégrées des diodes de détect ion 12 en siliciure de platine . Le plus souvent une couche d 'oxyde de silicium 14 recouvre la face arrière du substrat pour faciliter l'introduction sans réflexion du rayonnement infrarouge arrivant par la face arrière . In FIG. 1, which very schematically represents a conventional infrared detector (either monolithic or hybrid), a silicon substrate has been designated by 10, on the front face of which are integrated silicon detection ion diodes 12 platinum. Most often a layer of silicon oxide 14 covers the rear face of the substrate to facilitate the introduction without reflection of the infrared radiation arriving from the rear face.
Le plus souvent aussi, les diodes d e siliciure de platine 12 sont elles-mêmes recouvertes par une sorte de cavité optique constituée par une couche d'oxyde de silicium 16 puis une couche d'aluminium 18 formant miroir pour renvoyer vers l'arrière le rayonnement lumineux après qu'il a traversé la diode ; étant donné que l'absorption dans la diode n'est guère que de 0 , 5%, on a en effet intérêt à faire passer le rayonnement deux fois à travers la diode grâce à ce miroir. L'épaisseur d e la couche 14 est de préférence choisie pour que le rayonnement lumineux réfléchi par le miroir soit approximativement en phase avec le rayonnement incident, aux environs du centre de la bande d e longueurs d'onde à détecter.  Most often also, the platinum silicide diodes 12 are themselves covered by a sort of optical cavity constituted by a layer of silicon oxide 16 then a layer of aluminum 18 forming a mirror to reflect the radiation towards the rear. bright after it has passed through the diode; since the absorption in the diode is hardly more than 0.5%, it is in fact advantageous to pass the radiation twice through the diode by means of this mirror. The thickness of the layer 14 is preferably chosen so that the light radiation reflected by the mirror is approximately in phase with the incident radiation, around the center of the band of wavelengths to be detected.
Là où le miroir est bien perpendiculaire au plan du substrat, le rayonnement est réfléchi perpendiculairement vers l'arrière et ressort du substrat.  Where the mirror is well perpendicular to the plane of the substrate, the radiation is reflected perpendicularly towards the rear and emerges from the substrate.
Mais là où des surfaces réfléchissantes obliques sont présentes , que ce soit des portions obliques du miroir ou d'autres surfaces inclinées ou toute aut re cause de réflexion oblique , une fraction importante de rayonnement peut être réémise vers l'arrière avec une incidence telle qu'elle ne puisse pas ressortir du substrat ; cette fraction est renvoyée à nouveau vers les diodes de siliciure de platine , ma is loin de l'endroit d'incidence initiale .  But where oblique reflecting surfaces are present, whether oblique portions of the mirror or other inclined surfaces or any other cause of oblique reflection, a significant fraction of radiation can be re-emitted backwards with an incidence such that 'it cannot come out of the substrate; this fraction is returned again to the platinum silicide diodes, but is far from the place of initial incidence.
Il faut comprendre que ce phénomène est d'autant plus important que le rayonnement réfléchi est peu atténué puisque l'atténuation dans les diodes de siliciure de platine n'est guère que de 1% après passage dans les deux sens .  It should be understood that this phenomenon is all the more important that the reflected radiation is little attenuated since the attenuation in the diodes of platinum silicide is hardly more than 1% after passing in both directions.
Si le substrat a par exemple une épaisseur de 500 micromètres et si une fraction importante de rayonnement est réfléchie par exemple avec une incidence d'environ 30° par rapport à la normale , il se produira une image parasite à environ 500 micromètres de distance de l'image principale, c'est-à-dire à une vingtaine de pixels de distance pour des dimensions de pixels de l'ordre de 25 micromètres. If the substrate has for example a thickness of 500 micrometers and if a significant fraction of radiation is reflected for example with an incidence of approximately 30 ° compared to normal, a spurious image will be produced at approximately 500 micrometers away from the main image, that is to say about twenty pixels apart for pixel dimensions of the order of 25 micrometers.
Selon l'invention, on propose de réduire l'épaisseur du substrat jusqu'à des dimensions de quelques dizaines de micromètres, et de préférence même jusqu'à une dizaine de micromètres seulement, de sorte que les mêmes faisceaux réfléchis obliquement créeront une image parasite tres près de l'image principale (c'est-à-dire qu'en fait elles atténueront légèrement le contraste sans créer vraiment d'image parasite).  According to the invention, it is proposed to reduce the thickness of the substrate to dimensions of a few tens of micrometers, and preferably even up to only ten micrometers, so that the same beams reflected obliquely will create a spurious image. very close to the main image (that is to say that in fact they will slightly attenuate the contrast without really creating a stray image).
La figure 2 représente schématiquement l'invention. De même qu'à la figure 1, les échelles fie dimensions ne sont pas respectées pour que la structure générale reste intelligible.  FIG. 2 schematically represents the invention. As in FIG. 1, the dimensional scales are not respected so that the general structure remains intelligible.
Le substrat de silicium 10 est très fortement aminci par l'arrière, au moins en regard des diodes de détection (c'est-à-dire pas forcément à la périphérie du substrat). L'épaisseur finale est inférieure à environ 50 micromètres; elle est en principe comprise entre environ 50 micromètres et environ 10 micromètres.  The silicon substrate 10 is very greatly thinned from the rear, at least opposite the detection diodes (that is to say not necessarily at the periphery of the substrate). The final thickness is less than about 50 micrometers; it is in principle between about 50 micrometers and about 10 micrometers.
L'amincissement est effectué après intégration des éléments photosensibles (diodes au siliciure de platine notamment) dans un substrat semiconducteurs dont l'épaisseur est au départ par exemple de 250 à 500 micromètres. L'intégration des éléments photosensibles peut être effectuée comme dans la technique antérieure (techniques d'intégration monolithique sur silicium).  The thinning is carried out after integration of the photosensitive elements (diodes with platinum silicide in particular) in a semiconductor substrate whose thickness is initially for example from 250 to 500 micrometers. The integration of photosensitive elements can be carried out as in the prior art (techniques of monolithic integration on silicon).
L'amincissement peut être uniquement mécanique. par rodage. Dans ce cas. la totalité de la face arrière est évidemment amincie. On peut atteindre ainsi une épaisseur qui est par exemple d'environ 50 micromètres Cet amincissement mécanique peut être complété par une attaque chimique dans un bain de gravure attaquant le silicium, à condition bien entendu de protéger la face avant du substrat. On peut alors amincir le substrat jusqu'à atteindre une épaisseur d'une dizaine de micromètres. L'amélioration de la résolution est alors encore meilleure mais bien sûr la fabrication implique une étape de plus , cette étape étant assez délicate . L'amincissement chimique est fait principalement en regard des diodes et de préférence pas à la périphérie du substrat. Thinning can only be mechanical. by running-in. In that case. the entire rear face is obviously thinned. We can thus reach a thickness which is for example around 50 micrometers. This mechanical thinning can be supplemented by a chemical attack in an etching bath attacking the silicon, on condition of course of protecting the front face of the substrate. We can then thin the substrate until it reaches a thickness of ten micrometers. The improvement in resolution is then still better but of course the manufacturing involves one more step, this step being quite delicate. The chemical thinning is done mainly opposite the diodes and preferably not at the periphery of the substrate.
Etant donné que le substrat portant les diodes est en général destiné à être monté sur un autre substrat , on fera le montage sur l'autre substrat avant de procéder à l'amincissement , afin qu'il n'y ait pas de problème de tenue mécanique du substrat aminci.  Since the substrate carrying the diodes is generally intended to be mounted on another substrate, the assembly will be made on the other substrate before thinning, so that there is no problem of behavior mechanics of the thinned substrate.
Dans le cas d'un détecteur infrarouge hybride , la technologie est en général la suivante : le substrat portant les diodes de détection est soudé sur un subst rat portant le multiplexeur , par l'intermédiaire de billes d'indium qui font correspondre à chaque diode un élément du multiplexeur placé en vis-à-vis de cette diode . On procède donc à l'amincissement du substrat après assemblage des deux substrats ; s'il y a attaque chimique, il faut protéger notamment les connexions par billes d'indium et le deuxième substrat .  In the case of a hybrid infrared detector, the technology is generally as follows: the substrate carrying the detection diodes is welded to a subst rat carrying the multiplexer, by means of indium balls which correspond to each diode an element of the multiplexer placed opposite this diode. We therefore proceed with the thinning of the substrate after assembly of the two substrates; if there is chemical attack, it is necessary to protect in particular the connections by indium balls and the second substrate.
Dans le cas d'un détecteur infrarouge monolithique , on prévoit en général (pour des raisons de refroidissement et de tenue à l'environnement) que les détecteurs sont montés sur un substrat de céramique et connectés élect riquement à des plages conductrices de ce substrat . Le substrat est alors collé par sa face avant (portant les diodes de détection ) sur la céramique puisque la face arrière doit rester libre pour une illumination par l'arrière . Dans ce cas encore , on proecde à l'amincissement , aussi bien mécanique que chimique , après le montage du détecteur sur cette céramique .  In the case of a monolithic infrared detector, it is generally provided (for cooling and environmental reasons) that the detectors are mounted on a ceramic substrate and electrically connected to conductive pads of this substrate. The substrate is then bonded by its front face (carrying the detection diodes) to the ceramic since the rear face must remain free for illumination from the rear. In this case again, the thinning, both mechanical and chemical, is carried out after the detector has been mounted on this ceramic.
Dans les deux cas , il est souhaitable de déposer une couche d'adaptation antireflet 14 sur la face arrière amincie , pour faciliter la pénétration du rayonnement infrarouge sous incidence normale . Pour une longueur d'onde d'environ 4 micromètres , on prendra de préférence une couche d'oxyde de silicium d'épaisseur 700 nanomètres environ ; l'oxyde de silicium est choisi pour la raison qu'il a un indice intermédiaire entre celui de l'air et celui du silicium. In both cases, it is desirable to deposit an anti-reflection adaptation layer 14 on the thinned rear face, to facilitate the penetration of infrared radiation under normal incidence. For a wavelength of approximately 4 micrometers, it is preferable to take a layer of silicon oxide with a thickness of approximately 700 nanometers; silicon oxide is chosen for the reason that it has an intermediate index between that of air and that of silicon.
Les figures 3 à 5 représentent à t itre d'exemple les étapes de réalisation dans le cas d 'un détecteur matriciel monolithique (c'est-à-dire portant non seulement les éléments photosensibles mais aussi une circuiterie d e multiplexage pour recueillir et traiter les charges élect riques engendrées dans chaque élément photosensible) .  FIGS. 3 to 5 show by way of example the production steps in the case of a monolithic matrix detector (that is to say carrying not only the photosensitive elements but also a multiplexing circuitry for collecting and processing the electric charges generated in each photosensitive element).
Le substrat de silicium 10 portant les diodes et le reste de la circuiterie est soudé face contre face contre un substrat céramique (montage "flip chip") , des plots de connexion 20 sur la face avant du substrat 10 étant collés par une colle conductrice sur des plages conductrices 22 (sérigraphiées ou photogravées) d'un substrat de céramique 30. Ce substrat est destiné principalement au refroidissement du détecteur mais il servira aussi à assurer la rigidité du détecteur- après amincissement . Figure 3.  The silicon substrate 10 carrying the diodes and the rest of the circuitry is soldered face against face against a ceramic substrate ("flip chip" mounting), connection pads 20 on the front face of the substrate 10 being bonded with a conductive adhesive on conductive pads 22 (screen-printed or photo-etched) of a ceramic substrate 30. This substrate is mainly intended for cooling the detector but it will also serve to ensure the rigidity of the detector after thinning. Figure 3.
Le collage de la puce avec retournement peut aussi être effectué par une technique d'hybridation dans laquelle des microbilles (d'indium) formées sur le substrat de silicium sont soudées sur des plages métalliques formées sur le substrat de céramique . C'est d'ailleurs cette dernière technique que l'on utilisera si le deuxième substrat n'est pas un substrat de céramique et est plutôt un substrat fie silicium portant la circuiterie de multiplexage associée au détecteur .  Bonding of the chip with inversion can also be carried out by a hybridization technique in which microbeads (of indium) formed on the silicon substrate are welded on metal areas formed on the ceramic substrate. It is moreover this latter technique that will be used if the second substrate is not a ceramic substrate and is rather a silicon-based substrate carrying the multiplexing circuitry associated with the detector.
Un rodage mécanique de la face arrière du substrat de siliicum 10 est alors effectué (figure 1 ) ; l'épaisseur initiale el est par exemple de 250 ou 500 micromètres : l'épaisseur finale e2 est de quelques dizaines de micromèt res , de préférence environ 50 micromètres .  A mechanical running-in of the rear face of the siliicum substrate 10 is then carried out (FIG. 1); the initial thickness el is for example 250 or 500 micrometers: the final thickness e2 is a few tens of micrometers, preferably about 50 micrometers.
Un amincissement chimique peut alors compléter le rodage pour descendre à une épaisseur e3 de l 'ordre de la dizaine de micromètres (figure 5) . Pour cela on commence par protéger les bords du substrat par une couche de résine 26 résistant aux agents d'attaque chimique du silicium , de manière que les connexions entre la puce 10 et le substrat de céramique ne soient pas endommagées . A chemical thinning can then complete the running-in to descend to a thickness e3 of the order of ten micrometers (FIG. 5). For this, we start by protecting the edges of the substrate with a layer of resin 26 resistant to the chemical attack agents of silicon, so that the connections between the chip 10 and the ceramic substrate are not damaged.
La solution d'attaque est composée d 'acide acétique , acide fluorhydrique et acide nitrique . L'acide nitrique oxyde le silicium , l'acide flourhydrique dissous l'oxyde , et la concentration en acide acétique permet fie choisir la vitesse d'attaque .  The attack solution is composed of acetic acid, hydrofluoric acid and nitric acid. Nitric acid oxidizes silicon, dissolved hydrofluoric acid, and the concentration of acetic acid allows you to choose the attack speed.
Si la solution est plus riche en acide fluorhydrique , l'attaque sera d'autant plus rapide que le dopage du substrat sera plus élevé . Si la solution est plus riche en acide nitrique l'état de surface après attaque est plus homogène . On a donc avantage à procéder en plusieurs étapes , avec d'abord une solution riche en acide fluorydrique puis u ne solut ion riche en acide nitrique .  If the solution is richer in hydrofluoric acid, the attack will be all the more rapid as the doping of the substrate will be higher. If the solution is richer in nitric acid, the surface condition after attack is more homogeneous. It is therefore advantageous to proceed in several stages, first with a solution rich in fluororyric acid and then a solution rich in nitric acid.
Si le détecteur est réalisé sur un substrat épitaxié P/P+ , c'est-à-dire un substrat principalemen t d e t ype P+ portant une couche épitaxiée peu dopée de type P d'une dizaine de micromètres d'épaisseur par exemple , il sera facile d'éliminer toute la région P+ et de ne garder que la part ie épitaxiée , en profitant de la sélectivité de gravure de l 'acide fluorhydrique en fonction du dopage .  If the detector is produced on an epitaxial substrate P / P +, that is to say a substrate mainly tdet ype P + carrying a lightly doped epitaxial layer of type P of ten micrometers thick for example, it will be easy to eliminate the entire P + region and to keep only the epitaxial part, taking advantage of the selectivity of etching of hydrofluoric acid as a function of doping.
La couche antireflet 14 peut être déposée ensuite .  The anti-reflective layer 14 can then be deposited.
L'invention a été décrite en détail â propos d e détecteurs infrarouges dont les éléments photosensibles sont des diodes schottky au siliciure de platine sur silicium . On comprendra cependant qu'elle serait transposable a d es détecteurs infrarouges dont les éléments photosensibles sont différents , étant donné que le problème ne vient pas spécialement du fait de ces diodes mais bien plutôt de réflexions parasites sous incidence non normale .  The invention has been described in detail with regard to infrared detectors, the photosensitive elements of which are schottky diodes with platinum silicide on silicon. It will however be understood that it would be transposable to infrared detectors whose photosensitive elements are different, given that the problem does not come specially from these diodes but rather from parasitic reflections under non-normal incidence.

Claims

REVENDICATIONS
1. Détecteur infrarouge comportant un ensemble d'éléments photosensibles en infrarouge (12) constitués par des diodes Schottky et intégrés sur la face avant d'un substrat semiconducteur (10), caractérisé en ce que le substrat semiconducteur a une épaisseur inférieure à environ 50 micromètres, au moins en regard d'une région comprenant les éléments photosensibles. 1. Infrared detector comprising a set of infrared photosensitive elements (12) constituted by Schottky diodes and integrated on the front face of a semiconductor substrate (10), characterized in that the semiconductor substrate has a thickness of less than about 50 micrometers, at least with respect to a region comprising the photosensitive elements.
2. Détecteur selon la revendication 1, caractérisé en ce que le substrat est aminci sur toute sa face arrière. 2. Detector according to claim 1, characterized in that the substrate is thinned over its entire rear face.
3. Détecteur selon la revendication 1, caractérisé en ce que le substrat est plus mince au centre qu'à sa périphérie. 3. Detector according to claim 1, characterized in that the substrate is thinner in the center than at its periphery.
4. Détecteur selon l'une des revendications précédentes, caractérisé en ce que les éléments photosensibles sont des diodes schottky au siliciure de platine sur silicium. 4. Detector according to one of the preceding claims, characterized in that the photosensitive elements are schottky diodes with platinum silicide on silicon.
5. Détecteur selon l'une des revendications précédentes, caractérisé en ce que substrat semiconducteur est recouvert sur sa face arrière d'une couche d'adaptation antireflet (14). 5. Detector according to one of the preceding claims, characterized in that the semiconductor substrate is covered on its rear face with an anti-reflection adaptation layer (14).
6. Détecteur selon l'une des revendications 1 à 5, caractérisé en ce que les éléments photosensibles (12) sont recouverts d'une couche formant miroir (18) permettant à un rayonnement issu de la face arrière de passer deux fois dans les éléments photosensibles. 6. Detector according to one of claims 1 to 5, characterized in that the photosensitive elements (12) are covered with a mirror layer (18) allowing radiation from the rear face to pass twice through the elements photosensitive.
7. Détecteur selon l'une des revendications 1 à 6. caractérisé en ce que le substrat (10) est fixé par sa face avant sur un autre substrat (30). 7. Detector according to one of claims 1 to 6. characterized in that the substrate (10) is fixed by its front face to another substrate (30).
8. Procédé de réalisation d'un détecteur infrarouge , caractérisé en ce qu'on réalise selon une t echnique d'intégration monolithique un ensemble d'éléments photosensibles ( 12) sur la face avant d'un substrat semiconducteur ( 10) et en ce qu'on amincit ensuite le substrat par sa face arrière jusqu'à une épaisseur inférieure à environ 50 micromètres . 8. Method for making an infrared detector, characterized in that a set of monolithic integration techniques is used to produce a set of photosensitive elements (12) on the front face of a semiconductor substrate (10) and in that that the substrate is then thinned by its rear face to a thickness less than about 50 micrometers.
9. Procédé selon la revendication 8 , caractérisé en ce que l'amincissement est effectué par rodage mécanique . 9. Method according to claim 8, characterized in that the thinning is carried out by mechanical running-in.
10. Procédé selon la revendication 9 , caractérisé en ce que l'amincissement est complété , après rodage mécanique , par une attaque chimique , jusqu 'à une épaisseur d o substrat comprise entre 50 et 10 micromètres environ . 10. Method according to claim 9, characterized in that the thinning is completed, after mechanical running-in, by a chemical attack, up to a thickness of substrate between 50 and 10 micrometers approximately.
11. Procédé selon la revendication 10. caractérisé en ce que l'attaque chimique est effectuée au centre mais pas à la périphérie du substrat. 11. Method according to claim 10. characterized in that the chemical attack is carried out in the center but not at the periphery of the substrate.
12. Procédé selon l'une des revendications 8 à 11 , caractérisé en ce que l'amincissement du substrat est effectué après montage du substrat par sa face avant sur un autre substrat (30) . 12. Method according to one of claims 8 to 11, characterized in that the thinning of the substrate is carried out after mounting the substrate by its front face on another substrate (30).
PCT/FR1991/000713 1990-09-07 1991-09-06 Infrared detector with a thin substrate and method for producing same WO1992004737A1 (en)

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FR9011123A FR2666690B1 (en) 1990-09-07 1990-09-07 INFRARED DETECTOR WITH THINNED SUBSTRATE AND MANUFACTURING METHOD.
FR90/11123 1990-09-07

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Citations (3)

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Publication number Priority date Publication date Assignee Title
US4060822A (en) * 1973-06-27 1977-11-29 Siemens Aktiengesellschaft Strip type radiation detector and method of making same
EP0279492A1 (en) * 1987-02-16 1988-08-24 Koninklijke Philips Electronics N.V. Radiation-sensitive semiconductor device
US4782028A (en) * 1987-08-27 1988-11-01 Santa Barbara Research Center Process methodology for two-sided fabrication of devices on thinned silicon

Patent Citations (3)

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Publication number Priority date Publication date Assignee Title
US4060822A (en) * 1973-06-27 1977-11-29 Siemens Aktiengesellschaft Strip type radiation detector and method of making same
EP0279492A1 (en) * 1987-02-16 1988-08-24 Koninklijke Philips Electronics N.V. Radiation-sensitive semiconductor device
US4782028A (en) * 1987-08-27 1988-11-01 Santa Barbara Research Center Process methodology for two-sided fabrication of devices on thinned silicon

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Title
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IEEE TRANSACTIONS ON ELECTRON DEVICES. vol. 37, no. 3, Mars 1990, NEW YORK, US pages 629 - 635; K. KONUMA ET AL.: '324 x 487 Schottky-barrier infrared imager' voir le document en entier *

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EP0500878A1 (en) 1992-09-02
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FR2666690A1 (en) 1992-03-13

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