WO2003028077A1 - Method for thinning a silicon wafer - Google Patents

Method for thinning a silicon wafer Download PDF

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Publication number
WO2003028077A1
WO2003028077A1 PCT/FR2002/003257 FR0203257W WO03028077A1 WO 2003028077 A1 WO2003028077 A1 WO 2003028077A1 FR 0203257 W FR0203257 W FR 0203257W WO 03028077 A1 WO03028077 A1 WO 03028077A1
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WIPO (PCT)
Prior art keywords
etching
wafer
chemical etching
thinning
potash
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PCT/FR2002/003257
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French (fr)
Inventor
Christian Corriol
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Stmicroelectronics Sa
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Publication of WO2003028077A1 publication Critical patent/WO2003028077A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
    • H01L21/302—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to change their surface-physical characteristics or shape, e.g. etching, polishing, cutting
    • H01L21/306—Chemical or electrical treatment, e.g. electrolytic etching
    • H01L21/30604—Chemical etching
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
    • H01L21/302—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to change their surface-physical characteristics or shape, e.g. etching, polishing, cutting
    • H01L21/304—Mechanical treatment, e.g. grinding, polishing, cutting
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02041—Cleaning
    • H01L21/02043—Cleaning before device manufacture, i.e. Begin-Of-Line process
    • H01L21/02052—Wet cleaning only
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
    • H01L21/77—Manufacture or treatment of devices consisting of a plurality of solid state components or integrated circuits formed in, or on, a common substrate
    • H01L21/78—Manufacture or treatment of devices consisting of a plurality of solid state components or integrated circuits formed in, or on, a common substrate with subsequent division of the substrate into plural individual devices
    • H01L21/782—Manufacture or treatment of devices consisting of a plurality of solid state components or integrated circuits formed in, or on, a common substrate with subsequent division of the substrate into plural individual devices to produce devices, each consisting of a single circuit element
    • H01L21/784—Manufacture or treatment of devices consisting of a plurality of solid state components or integrated circuits formed in, or on, a common substrate with subsequent division of the substrate into plural individual devices to produce devices, each consisting of a single circuit element the substrate being a semiconductor body

Definitions

  • the present invention relates to a method for thinning a silicon wafer, to an integrated circuit produced on such a wafer, and to a smart card incorporating such a circuit.
  • the thinning of a silicon wafer (also called “wafer” in English) has hitherto been carried out by mechanical polishing of the rear face of this wafer, generally after the production of integrated circuits on the front face.
  • the objective of this thinning is to be able to put the integrated circuits in thin cases.
  • Said thinning finds, for example, an application in the field of smart cards, which must have a certain flexibility, this flexibility being all the less important as the integrated circuits used in these cards have significant thicknesses.
  • the subject of the invention is a method for thinning a silicon wafer by etching its rear face, characterized in that it consists in carrying out a first partial etching by mechanical polishing followed by chemical etching.
  • the chemical etching is carried out in a potash solution or several potash solutions taken successively.
  • the potash solution (s) comprise ethylene glycol, and preferably the potash / ethylene glycol volume ratio is of the order of 10/1.
  • chemical etching is carried out at a temperature between 75 ° C and 85 ° C, and preferably between 79 ° C and 81 ° C.
  • the wafer Before carrying out the chemical etching, the wafer can be immersed without subsequent rinsing in a bath comprising at least one soaking compound chosen from alcohols or their mixtures, the alcohol preferably being chosen from isopropyl alcohol, ethanol and their mixture.
  • a layer of Borosilicagel and / or TiN on the front face of said wafer, so as to form therein at least one protective layer consisting essentially of TiN and / or BSG / TiN.
  • Another subject of the invention is an integrated circuit formed on a thinned silicon wafer according to the method of the invention.
  • a final object of the invention is a smart card characterized in that it comprises the integrated circuit as mentioned above.
  • the method typically applies to wafers made of monocrystalline silicon. Their initial thickness, before any treatment, is for example around 800 ⁇ m and their diameter is for example 150 mm (it is of course possible to treat wafers of different thickness and diameter).
  • the method according to the invention makes it possible to thin these plates by engraving their rear face (the circuits being conventionally produced on the front face) in order to obtain thin integrated circuits, for example of 180 ⁇ m, for applications such as cards. smart. In the example, a thinning of 620 ⁇ m is carried out.
  • the thickness of the plates is first partially reduced by a first etching by mechanical polishing, said plates having their greatest thickness, 800 ⁇ m in the example illustrated.
  • the first thinning takes place over a thickness of less than 620 ⁇ m, to reduce the thickness of the plates for example to 300 ⁇ m.
  • the remaining thickness is then thinned by chemical etching carried out by placing the plates in an etching solution, so as to obtain the desired final thickness.
  • the chemical etching will be carried out on a thickness of 120 ⁇ m to obtain a final thickness of 180 ⁇ m.
  • the method according to the invention makes it possible to improve the uniformity of the thinning, independently of the diameter of the wafer and with a very low breakage percentage.
  • chemical etching is less aggressive than mechanical polishing and has the advantage of being applied uniformly over the entire surface of the etched face. This gives particular interest to the process for easily, quickly and inexpensively treating platelets of large diameter.
  • the invention also has the advantage of eliminating the conventional chemical treatment step of relaxation of stress applied following etching by mechanical polishing in the prior art.
  • This additional step carried out previously to remove the microcracks created by mechanical stress during chemical polishing, is in the invention, carried out by the chemical etching step concurrently with its polishing role. Mechanical stresses caused by the chemical etching step- being lower than the mechanical stress of mechanical polishing, the risk of breakage of the pads is reduced.
  • the front face of the wafer is protected before implementing chemical etching, by depositing a protective layer, for example TiN, ' over a thickness of a few hundred nanometers, for example 400 nm.
  • a protective layer for example TiN
  • the thickness of this layer will be chosen as a function of the duration of the chemical etching so that the layer is always present on the wafer at the end of the etching. In fact, if said layer is subjected to attack by the etching solution, it will decrease in thickness.
  • a first planarizing sublayer for example made of BSG.
  • the resistance of the protective layer to chemical etching will be all the more uniform on the wafer that this layer will have a substantially constant thickness over the whole of the wafer. Otherwise, there may be a risk of delamination at places where the layer has a lower thickness.
  • the addition of this sub-layer makes it possible to minimize the heights of the reliefs on the front face of the wafer before deposition of the protective layer itself.
  • this sublayer borosilicagel (BSG) at 8% of Boron, typically a few hundred nanometers, for example 800 nm.
  • the thickness of this sub-layer will be determined according to the reliefs present on the front face.
  • a solution of hydrofluoric and / or nitric acid may be used for chemical etching.
  • a potash solution is used, which makes it possible to obtain a lower etching speed and to more easily control the thickness of etched silicon and the uniformity of etching.
  • a solution of 500 cm 3 of 6N potassium hydroxide to treat a wafer, take a solution of 500 cm 3 of 6N potassium hydroxide.
  • the potash will be mixed with ethylene glycol in a volume ratio 10/1, ie in the example with 50 cm 3 of ethylene glycol, which makes it possible to improve the uniformity of the etching.
  • the potash concentration can be modified according to the desired etching thickness and the time required to obtain it. In practice, a less concentrated potash solution decreases the etching speed. A more concentrated solution of potassium hydroxide instead allows to reduce the etching time, but it has the disadvantage of increasing the risk of detachment of the layer 'protection, said delamination occurring on the edge of the wafer which does not include protective layer, since this layer is only arranged on the front face of the wafer.
  • the wafers Before being placed in the chemical etching solution, and just after mechanical polishing, the wafers can be immersed beforehand, and without subsequent rinsing, in a bath comprising isopropyl alcohol, ethanol or a mixture of the two, for a few seconds. This preliminary soaking allows a better uniformity of the chemical etching to be obtained subsequently.
  • a protective film consisting of alcohol on the surface of the wafer makes it possible to protect the silicon from oxygen in the air, thus avoiding the oxidation of the silicon on the surface before it is subjected to the bath of chemical etching. Without a protective film, the surface of the silicon wafer will oxidize in the open air, and a thin film of about a few angstroms of Si0 2 will be created .
  • the bath formed by the alcohol also makes it possible to weaken the film of SiO 2 , if such a film has already formed, again improving the uniformity of the subsequent chemical etching. Failing this, in fact, the etching speed is different depending on whether it is carried out on places on the wafer covered or not with Si0 2 , or if the Si0 2 film does not have a substantially identical thickness on the whole. of the brochure.
  • the plate is placed in the bath comprising the potassium hydroxide and ethylene glycol solution.
  • the main chemical reaction that takes place during chemical etching is as follows:
  • the front face of the wafer is turned towards the bottom of the bath and the rear face towards the surface, in order to facilitate the release of gas resulting from the chemical attack.
  • the wafer can, according to another more suitable embodiment in an industrial setting, be arranged vertically in the bath, this q ⁇ i also facilitates the release of gas.
  • a single bath may be used for chemical etching, periodically or not, renewing the chemical solution of the bath, or immerse the wafer in several baths successively.
  • the etching speed is around 2 ⁇ m / min. After 2 hours of engraving, this speed goes to approximately 1.6 ⁇ m / min, and after 3 hours of engraving at 1.2 ⁇ m / min.
  • a mechanical polishing of the order of 100 ⁇ m per minute considering a mechanical polishing of the order of 100 ⁇ m per minute, one can choose to carry out a mechanical polishing of 5 min followed by an etching chemical for 1 hour.
  • the wafer is rinsed, for example with deionized water, in order to eliminate the potassium which may remain on the wafer and to prevent the etching from continuing after the wafers have been removed from the etching solution.
  • the temperature of the etching bath (es) used varies very little during the process.
  • This temperature will preferably be in a range of 75 ° C to 85 ° C.
  • a temperature of around 80 ° C. has been chosen, the latter value representing a good compromise between etching speed and etching uniformity.
  • the choice of a lower temperature allows better control of the process, the etching speed being lower the lower the temperature.
  • it has the drawback of increasing the time necessary to obtain an etching on a given thickness. Choosing a high temperature can reduce this time.
  • the etching speed increases very strongly with temperature, which increases the risk of non-uniformity of this etching as well as the risk of the protective layer peeling off.
  • the control of the etching thickness is therefore more delicate.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Manufacturing & Machinery (AREA)
  • Computer Hardware Design (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Power Engineering (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Weting (AREA)

Abstract

The invention concerns a method for thinning a silicon wafer by etching its rear surface. The method consists in carrying out a first partial etching, by mechanical polishing, followed by a chemical etching in one or several solutions, preferably of potassium. The invention also concerns an integrated circuit formed on a wafer thinned by said method, and smart cards comprising such an integrated circuit.

Description

PROCEDE D'AMINCISSEMENT D'UNE PLAQUETTE DE SILICIUM METHOD FOR SLIMMING A SILICON WAFER
La présente invention se rapporte à un procédé d'amincissement d'une plaquette de silicium, à un circuit intégré réalisé sur une telle plaquette, et à une carte à puce intégrant un tel circuit. L'amincissement d'une plaquette de silicium (encore appelée « wafer » en anglais) est réalisé jusqu'ici par polissage mécanique de la face arrière de cette plaquette, généralement après réalisation de circuits intégrés sur la face avant. L'objectif de cet amincissement est de pouvoir mettre les circuits intégrés dans des boîtiers de faible épaisseur. Ledit amincissement trouve par exemple une application dans le domaine des cartes à puce, qui doivent présenter une certaine flexibilité, cette flexibilité étant d'autant moins importante que les circuits intégrés utilisés dans ces cartes présentent des épaisseurs importantes.The present invention relates to a method for thinning a silicon wafer, to an integrated circuit produced on such a wafer, and to a smart card incorporating such a circuit. The thinning of a silicon wafer (also called “wafer” in English) has hitherto been carried out by mechanical polishing of the rear face of this wafer, generally after the production of integrated circuits on the front face. The objective of this thinning is to be able to put the integrated circuits in thin cases. Said thinning finds, for example, an application in the field of smart cards, which must have a certain flexibility, this flexibility being all the less important as the integrated circuits used in these cards have significant thicknesses.
Une telle méthode d' amincissement mécanique est certes rapide pour obtenir le résultat final souhaité, mais entraîne l'apparition de microfissures sur la plaquette, du fait de l'importance du stress mécanique subi par celle-ci.Such a method of mechanical thinning is certainly quick to obtain the desired final result, but causes the appearance of microcracks on the wafer, because of the importance of the mechanical stress undergone by the latter.
• En outre, l'amincissement de la plaquette effectué par voie mécanique aboutit à une uniformité de mauvaise qualité de la face arrière traitée et ceci d'autant plus que la plaquette présente un diamètre important. • In addition, the thinning of the wafer carried out mechanically results in poor quality uniformity of the rear face treated and this all the more that the wafer has a large diameter.
Enfin, l'utilisation d'un amincissement de nature mécanique provoque un pourcentage élevé de casse des plaquettes, du fait de l'importance des contraintes appliquées sur la face arrière des plaquettes.Finally, the use of mechanical thinning causes a high percentage of breakage of platelets, due to the importance of the stresses applied to the rear face of the platelets.
Aussi, il subsiste le besoin de disposer d'un procédé d'amincissement d'une plaquette, de mise en œuvre simple, d'exécution suffisamment rapide, et n'entraînant pas les inconvénients mentionnés précédemment, dans le but de disposer de circuits de faible épaisseur.Also, there remains the need to have a method of thinning a wafer, of simple implementation, of sufficiently rapid execution, and not causing the drawbacks mentioned above, in order to have circuits for thin.
L'invention a pour objet un procédé pour amincir une plaquette de silicium par gravure de sa face arrière, caractérisé en ce qu'il consiste à réaliser une première gravure partielle par polissage mécanique suivie d'une gravure chimique.The subject of the invention is a method for thinning a silicon wafer by etching its rear face, characterized in that it consists in carrying out a first partial etching by mechanical polishing followed by chemical etching.
De préférence, la gravure chimique est réalisée dans une solution de potasse ou plusieurs solutions de potasse prises successivement.Preferably, the chemical etching is carried out in a potash solution or several potash solutions taken successively.
Selon un mode de réalisation, la ou les solutions de potasse comprennent de l' éthylèneglycol, et de préférence le rapport volumique potasse/éthylèneglycol est de l'ordre de 10/1.According to one embodiment, the potash solution (s) comprise ethylene glycol, and preferably the potash / ethylene glycol volume ratio is of the order of 10/1.
Selon l'invention, la gravure chimique est effectuée à une température comprise entre 75 °C et 85°C, et de préférence entre 79°C et 81°C.According to the invention, chemical etching is carried out at a temperature between 75 ° C and 85 ° C, and preferably between 79 ° C and 81 ° C.
Avant d'effectuer la gravure chimique, la plaquette peut être immergée sans rinçage ultérieur dans un bain comprenant au moins un composé de trempage choisi parmi les alcools ou leurs mélanges, l'alcool étant de préférence choisi parmi l'alcool isopropylique, l'éthanol et leur mélange. Avant d' effectuer une gravure chimique de la face arrière de la plaquette, on peut déposer une couche de Borosilicagel et/ou de TiN sur la face avant de ladite plaquette, de manière à y former au moins une couche de protection constituée essentiellement en TiN et/ou en BSG/TiN. L'invention a encore pour objet un circuit intégré formé sur une plaquette de .silicium amincie selon le procédé de l'invention.Before carrying out the chemical etching, the wafer can be immersed without subsequent rinsing in a bath comprising at least one soaking compound chosen from alcohols or their mixtures, the alcohol preferably being chosen from isopropyl alcohol, ethanol and their mixture. Before performing a chemical etching of the rear face of the wafer, a layer of Borosilicagel and / or TiN on the front face of said wafer, so as to form therein at least one protective layer consisting essentially of TiN and / or BSG / TiN. Another subject of the invention is an integrated circuit formed on a thinned silicon wafer according to the method of the invention.
Enfin un dernier objet de l'invention est une carte à puce caractérisé en ce qu'elle comprend le circuit intégré tel que mentionné ci-dessus.Finally, a final object of the invention is a smart card characterized in that it comprises the integrated circuit as mentioned above.
L'invention va maintenant être détaillée à l'aide d'un exemple de réalisation qui suit et qui n'est nullement limitatif.The invention will now be detailed with the aid of an embodiment which follows and which is in no way limiting.
Le procédé s'applique, typiquement, à des plaquettes constituées de silicium monocristallin. Leur épaisseur initiale, avant tout traitement, est par exemple d'environ 800 μm et leur diamètre est par exemple de 150 mm (on pourra bien entendu traiter des plaquettes d'épaisseur et de diamètre différents). Le procédé selon l'invention permet d'amincir ces plaquettes en gravant leur face arrière (les circuits étant réalisés classiquement sur la face avant) pour obtenir des circuits intégrés de faible épaisseur, par exemple de 180 μm, pour des applications telles que les cartes à puce. On réalise dans l'exemple un amincissement de 620 μm.The method typically applies to wafers made of monocrystalline silicon. Their initial thickness, before any treatment, is for example around 800 μm and their diameter is for example 150 mm (it is of course possible to treat wafers of different thickness and diameter). The method according to the invention makes it possible to thin these plates by engraving their rear face (the circuits being conventionally produced on the front face) in order to obtain thin integrated circuits, for example of 180 μm, for applications such as cards. smart. In the example, a thinning of 620 μm is carried out.
Selon l'invention, on réduit en premier lieu partiellement l'épaisseur des plaquettes par une première gravure par polissage mécanique, lesdites plaquettes ayant leur épaisseur la plus importante, 800 μm dans l'exemple illustré. Le premier amincissement s'effectue sur une épaisseur inférieure à 620 μm, pour ramener l'épaisseur des plaquettes par exemple à 300 μm. On amincit ensuite l'épaisseur restante par une gravure chimique réalisée en disposant les plaquettes dans une solution de gravure, de sorte à obtenir l'épaisseur finale souhaitée. Dans l'exemple illustré, la gravure chimique sera réalisée sur une épaisseur de 120 μm pour obtenir une épaisseur finale de 180 μm. Comparativement à la solution antérieure consistant à réaliser l'intégralité de l'amincissement par polissage mécanique de la face arrière, le procédé selon l'invention permet d'améliorer l'uniformité de l'amincissement, indépendamment du diamètre de la plaquette et avec un très faible pourcentage de casse. En effet, la gravure chimique est moins agressive qu'un polissage mécanique et a l'avantage de s'appliquer uniformément sur toute la surface de la face gravée. Ceci donne un intérêt particulier au procédé pour traiter facilement, rapidement et à moindre coût des plaquettes de diamètre important.According to the invention, the thickness of the plates is first partially reduced by a first etching by mechanical polishing, said plates having their greatest thickness, 800 μm in the example illustrated. The first thinning takes place over a thickness of less than 620 μm, to reduce the thickness of the plates for example to 300 μm. The remaining thickness is then thinned by chemical etching carried out by placing the plates in an etching solution, so as to obtain the desired final thickness. In the example illustrated, the chemical etching will be carried out on a thickness of 120 μm to obtain a final thickness of 180 μm. Compared to the previous solution consisting in carrying out all of the thinning by mechanical polishing of the rear face, the method according to the invention makes it possible to improve the uniformity of the thinning, independently of the diameter of the wafer and with a very low breakage percentage. In fact, chemical etching is less aggressive than mechanical polishing and has the advantage of being applied uniformly over the entire surface of the etched face. This gives particular interest to the process for easily, quickly and inexpensively treating platelets of large diameter.
L'invention présente encore comme avantage de supprimer l'étape classique de traitement chimique de relâchement de contrainte appliquée à la suite de la gravure par polissage mécanique dans l'art antérieur. Cette étape supplémentaire, réalisée antérieurement pour supprimer les microfissures créées par le stress mécanique lors du polissage chimique, est dans l'invention, réalisée par l'étape de gravure chimique concurremment à son rôle de polissage. Les contraintes mécaniques occasionnées par l'étape de gravure chimique- étant inférieures au stress mécanique du polissage mécanique, on diminue le risque de casse des plaquettes .The invention also has the advantage of eliminating the conventional chemical treatment step of relaxation of stress applied following etching by mechanical polishing in the prior art. This additional step, carried out previously to remove the microcracks created by mechanical stress during chemical polishing, is in the invention, carried out by the chemical etching step concurrently with its polishing role. Mechanical stresses caused by the chemical etching step- being lower than the mechanical stress of mechanical polishing, the risk of breakage of the pads is reduced.
Selon un mode de réalisation, on protège la face avant de la plaquette avant de mettre en œuvre la gravure chimique, par le dépôt d'une couche de protection, par exemple du TiN, ' sur une épaisseur de quelques centaines de nanomètres, par exemple 400 nm. L'épaisseur de cette couche sera choisie en fonction de la durée de la gravure chimique de sorte que la couche soit toujours présente sur la plaquette en fin de gravure. En effet, si la dite couche est soumise à une attaque par la solution de gravure, elle voit son épaisseur diminuer. On peut éventuellement déposer, antérieurement à la couche de protection en TiN, une première sous-couche aplanissante par exemple en BSG. En effet, la résistance de la couche de protection à la gravure chimique sera d' autant plus uniforme sur la plaquette que cette couche aura une épaisseur sensiblement constante sur l'ensemble de la plaquette. Dans le cas contraire, il peut exister un risque de décollement aux endroits où la couche a une épaisseur plus faible. Pour les plaquettes sur lesquelles on réalise des circuits présentant des reliefs prononcés, telles que les circuits comprenant des cellules mémoire par exemple, l'ajout de cette sous-couche permet de minimiser les hauteurs des reliefs sur la face avant de la plaquette avant le dépôt de la couche de protection proprement dite. A titre d'exemple, on pourra utiliser pour réaliser cette sous-couche du borosilicagel (BSG) à 8 % de Bore, typiquement de quelques centaines de nanomètres, par exemple 800 nm. L'épaisseur de cette sous-couche sera déterminée en fonction des reliefs présents sur la face avant. Pour la gravure chimique, on pourra utiliser une solution d'acide fluorhydrique et/ou nitrique.According to one embodiment, the front face of the wafer is protected before implementing chemical etching, by depositing a protective layer, for example TiN, ' over a thickness of a few hundred nanometers, for example 400 nm. The thickness of this layer will be chosen as a function of the duration of the chemical etching so that the layer is always present on the wafer at the end of the etching. In fact, if said layer is subjected to attack by the etching solution, it will decrease in thickness. It is optionally possible to deposit, prior to the protective layer of TiN, a first planarizing sublayer, for example made of BSG. Indeed, the resistance of the protective layer to chemical etching will be all the more uniform on the wafer that this layer will have a substantially constant thickness over the whole of the wafer. Otherwise, there may be a risk of delamination at places where the layer has a lower thickness. For wafers on which circuits with pronounced reliefs are produced, such as circuits comprising memory cells for example, the addition of this sub-layer makes it possible to minimize the heights of the reliefs on the front face of the wafer before deposition of the protective layer itself. For example, we can use to make this sublayer borosilicagel (BSG) at 8% of Boron, typically a few hundred nanometers, for example 800 nm. The thickness of this sub-layer will be determined according to the reliefs present on the front face. For chemical etching, a solution of hydrofluoric and / or nitric acid may be used.
• Dans une version préférée on utilise une solution de potasse, qui permet d'obtenir une vitesse de gravure moins élevée et de contrôler plus facilement l'épaisseur de silicium gravée et l'uniformité de la gravure. On pourra par exemple, pour traiter une plaquette, prendre une solution de 500 cm3 de potasse 6N. De préférence, la potasse sera mélangée avec de l' éthylèneglycol dans un rapport volumique 10/1, i.e. dans l'exemple avec 50 cm3 d' éthylèneglycol, ce qui permet d'améliorer l'uniformité de la gravure.• In a preferred version, a potash solution is used, which makes it possible to obtain a lower etching speed and to more easily control the thickness of etched silicon and the uniformity of etching. For example, to treat a wafer, take a solution of 500 cm 3 of 6N potassium hydroxide. Preferably, the potash will be mixed with ethylene glycol in a volume ratio 10/1, ie in the example with 50 cm 3 of ethylene glycol, which makes it possible to improve the uniformity of the etching.
La concentration de potassé pourra être modifiée en fonction de l'épaisseur de gravure recherchée et du temps nécessaire pour l'obtenir. En pratique, une solution de potasse moins concentrée diminue la vitesse de gravure. Une solution de potasse plus concentrée permet au contraire de diminuer le temps de gravure, mais elle présente l'inconvénient d'augmenter le risque de décollement de la couche 'de protection, ledit décollement se produisant sur la tranche de la plaquette qui ne comprend aucune couche de protection, puisque cette couche est uniquement disposée sur la face avant de la plaquette.The potash concentration can be modified according to the desired etching thickness and the time required to obtain it. In practice, a less concentrated potash solution decreases the etching speed. A more concentrated solution of potassium hydroxide instead allows to reduce the etching time, but it has the disadvantage of increasing the risk of detachment of the layer 'protection, said delamination occurring on the edge of the wafer which does not include protective layer, since this layer is only arranged on the front face of the wafer.
Avant d'être placées dans la solution de gravure chimique, et juste après le polissage mécanique, les plaquettes peuvent être immergées préalablement, et sans rinçage ultérieur, dans un bain comprenant de l'alcool isopropylique, de l'éthanol ou un mélange des deux, pendant quelques secondes. Ce trempage préalable permet d' obtenir ultérieurement une meilleure uniformité de la gravure chimique. La présence d'un film protecteur constitué par l'alcool sur la surface de la plaquette permet de protéger le silicium de l'oxygène de l'air, évitant ainsi l'oxydation du silicium en surface avant qu'elle soit soumise au bain de gravure chimique. Sans film protecteur, la surface de la plaquette constituée de silicium va s'oxyder à l'air libre, et il se créera une fine pellicule d'environ quelques angstroms de Si02. Le bain constitué par l'alcool permet également de fragiliser la pellicule de Si02, si une telle pellicule s'est déjà formée, améliorant là aussi l'uniformité de la gravure chimique ultérieure. A défaut, en effet, la vitesse de gravure est différente selon qu'elle est réalisée sur des endroits de la plaquette recouverts ou non de Si02, ou si la pellicule de Si02 n' a pas une épaisseur sensiblement identique sur l'ensemble de la plaquette.Before being placed in the chemical etching solution, and just after mechanical polishing, the wafers can be immersed beforehand, and without subsequent rinsing, in a bath comprising isopropyl alcohol, ethanol or a mixture of the two, for a few seconds. This preliminary soaking allows a better uniformity of the chemical etching to be obtained subsequently. The presence of a protective film consisting of alcohol on the surface of the wafer makes it possible to protect the silicon from oxygen in the air, thus avoiding the oxidation of the silicon on the surface before it is subjected to the bath of chemical etching. Without a protective film, the surface of the silicon wafer will oxidize in the open air, and a thin film of about a few angstroms of Si0 2 will be created . The bath formed by the alcohol also makes it possible to weaken the film of SiO 2 , if such a film has already formed, again improving the uniformity of the subsequent chemical etching. Failing this, in fact, the etching speed is different depending on whether it is carried out on places on the wafer covered or not with Si0 2 , or if the Si0 2 film does not have a substantially identical thickness on the whole. of the brochure.
Pour la réalisation de la gravure chimique, la plaquette est disposée dans le bain comprenant la solution de potasse et d' éthylèneglycol . La réaction chimique principale qui a lieu pendant la gravure chimique est la suivante :To carry out the chemical etching, the plate is placed in the bath comprising the potassium hydroxide and ethylene glycol solution. The main chemical reaction that takes place during chemical etching is as follows:
Si + 3H20 + K+OH" Si (OH)4 + 3/2 H2 + K Pour obtenir un rendement maximum de ce procédé de gravure chimique, il est préférable de ventiler le bain pour évacuer l'hydrogène formé (très dilué), ainsi que de filtrer en continu- le bain de potasse pour éliminer le potassium formé qui tombe dans le fond du bain au cours de la réaction chimique.Si + 3H 2 0 + K + OH " Si (OH) 4 + 3/2 H 2 + K To obtain maximum efficiency from this chemical etching process, it is preferable to ventilate the bath to evacuate the hydrogen formed (very diluted), as well as continuously filtering the potash bath to remove the potassium formed which falls to the bottom of the bath during the chemical reaction.
De préférence, la face avant de la plaquette est tournée vers le fond du bain et la face arrière vers la surface, afin de faciliter la dégagement gazeux issu de l'attaque chimique.Preferably, the front face of the wafer is turned towards the bottom of the bath and the rear face towards the surface, in order to facilitate the release of gas resulting from the chemical attack.
La plaquette peut, selon un autre mode de réalisation plus approprié dans un cadre industriel, être disposée verticalement dans le bain, ce qμi facilite également le dégagement gazeux.The wafer can, according to another more suitable embodiment in an industrial setting, be arranged vertically in the bath, this qμi also facilitates the release of gas.
En fonction de l'épaisseur de gravure souhaitée et du temps nécessaire à l'obtention du résultat recherché, on pourra utiliser un bain unique pour la gravure chimique en renouvelant périodiquement ou non la solution chimique du bain, ou immerger la plaquette dans plusieurs bains successivement. Dans ces deux derniers cas, en prenant soin de ne pas rincer et sécher les plaquettes entre chaque bain ou lors des renouvellements de la solution, on pourra se dispenser d' immerger les plaquettes dans une solution d' alcool intermédiaire, la protection de la plaquette étant obtenue par le film résiduel de solution de gravure qui reste sur les plaquettes.Depending on the desired etching thickness and the time required to obtain the desired result, a single bath may be used for chemical etching, periodically or not, renewing the chemical solution of the bath, or immerse the wafer in several baths successively. . In these last two cases, taking care not to rinse and dry the platelets between each bath or during the replenishments of the solution, one can dispense with immersing the platelets in an intermediate alcohol solution, protecting the platelet being obtained by the residual film of etching solution which remains on the wafers.
A titre indicatif, lorsque la gravure commence dans un bain vierge de toutes impuretés (c'est à dire n'ayant encore jamais été utilisé) et sans élimination des impuretés en cours de procédé, avec une température du bain maintenue à 80 °C +/- 1°C, la vitesse de gravure est d'environ 2 μm/min. Après 2 heures de gravure, cette vitesse passe à environ 1,6 μm/min, et après 3 heures de gravure à 1,2 μm/min. Dans l'exemple choisi d'une plaquette de 800 μm à amincir de 620 μm, en considérant un polissage mécanique de l'ordre de 100 μm par minute, on pourra choisir d'effectuer un polissage mécanique de 5 mn suivi d'une gravure chimique pendant 1 heure.As an indication, when the etching begins in a bath free of all impurities (ie having never been used before) and without removing impurities during the process, with a bath temperature maintained at 80 ° C + / - 1 ° C, the etching speed is around 2 μm / min. After 2 hours of engraving, this speed goes to approximately 1.6 μm / min, and after 3 hours of engraving at 1.2 μm / min. In the example chosen of a 800 μm wafer to be slimmed by 620 μm, considering a mechanical polishing of the order of 100 μm per minute, one can choose to carry out a mechanical polishing of 5 min followed by an etching chemical for 1 hour.
A l'issue de la gravure chimique, la plaquette est rincée, par exemple avec de l'eau déionisée, afin d'éliminer la potasse pouvant rester sur la plaquette et éviter que la gravure ne se 'prolonge une fois les plaquettes retirées de la solution de gravure.After chemical etching, the wafer is rinsed, for example with deionized water, in order to eliminate the potassium which may remain on the wafer and to prevent the etching from continuing after the wafers have been removed from the etching solution.
Afin de pouvoir contrôler la vitesse de gravure, il est souhaitable que la température du ou des bains de gravure utilisés varie très peu au cours du procédé. Cette température sera de préférence dans une plage de 75°C à 85°C. Dans une réalisation préférée, on a choisi une température d'environ 80 °C, cette dernière valeur représentant un bon compromis vitesse de gravure/uniformité de gravure. Le choix d'une température moins élevée permet un meilleur contrôle du procédé, la vitesse de gravure étant d'autant moins élevée que la température est basse. Elle présente par contre l'inconvénient d'augmenter le temps nécessaire pour obtenir une gravure sur une épaisseur donnée. Le choix d'une température élevée permet de réduire ce temps. Par contre, le vitesse de gravure augmente très fortement avec la température, ce qui augmente le risque de non uniformité de cette gravure ainsi que le risque de décollement de la couche de protection. En choisissant une température élevée, le contrôle de l'épaisseur de gravure est donc plus délicat. En dernier lieu lorsque la gravure chimique est terminée, on effectue la déprotection de la face avant des plaquettes à gravure par le retrait des couches de protection (TiN et BSG 8%) selon des procédés classiques, en utilisant par exemple du CHF3 ou du CF . Le cas échéant cette déprotection pourra n'être réalisée que partiellement, au niveau des plots de connexion des circuits. In order to be able to control the etching speed, it is desirable that the temperature of the etching bath (es) used varies very little during the process. This temperature will preferably be in a range of 75 ° C to 85 ° C. In a preferred embodiment, a temperature of around 80 ° C. has been chosen, the latter value representing a good compromise between etching speed and etching uniformity. The choice of a lower temperature allows better control of the process, the etching speed being lower the lower the temperature. On the other hand, it has the drawback of increasing the time necessary to obtain an etching on a given thickness. Choosing a high temperature can reduce this time. On the other hand, the etching speed increases very strongly with temperature, which increases the risk of non-uniformity of this etching as well as the risk of the protective layer peeling off. By choosing a high temperature, the control of the etching thickness is therefore more delicate. Finally when the chemical etching is finished, the front face of the etching plates is deprotected by removing the protective layers (TiN and BSG 8%) according to conventional methods, using for example CHF 3 or CF. If necessary, this deprotection may only be carried out partially, at the level of the connection pads of the circuits.

Claims

REVENDICATIONS
1. Procédé pour amincir une plaquette de silicium par gravure de sa face arrière, caractérisé en ce qu'il consiste à réaliser une première gravure partielle par polissage mécanique suivie d'une gravure chimique.1. Method for thinning a silicon wafer by etching its rear face, characterized in that it consists in carrying out a first partial etching by mechanical polishing followed by chemical etching.
2. Procédé selon la revendication 1, caractérisé en ce que la gravure chimique est réalisée dans une solution de potasse ou plusieurs solutions de potasse prises successivement.2. Method according to claim 1, characterized in that the chemical etching is carried out in a potash solution or several potash solutions taken successively.
3. Procédé selon la revendication 2, caractérisé en ce que la ou les solutions de potasse comprennent de 1' éthylèneglycol .3. Method according to claim 2, characterized in that the potash solution or solutions comprise ethylene glycol.
4. Procédé selon la revendication 3, caractérisé en ce que le rapport volumique potasse/éthylèneglycol est de 10/1.4. Method according to claim 3, characterized in that the potash / ethylene glycol volume ratio is 10/1.
5. Procédé selon l'une des revendications 2 à 4, caractérisé en ce que la gravure chimique est effectuée à une température comprise entre 75°C et 85°C, et de préférence entre 79°C et 81°C.5. Method according to one of claims 2 to 4, characterized in that the chemical etching is carried out at a temperature between 75 ° C and 85 ° C, and preferably between 79 ° C and 81 ° C.
6. Procédé selon l'une des revendications précédentes, caractérisé en ce qu'avant d'effectuer la gravure chimique, la plaquette est immergée sans rinçage successif dans un bain comprenant au moins un composé de trempage choisi parmi les alcools ou leurs mélanges .6. Method according to one of the preceding claims, characterized in that before performing the chemical etching, the wafer is immersed without successive rinsing in a bath comprising at least one soaking compound chosen from alcohols or their mixtures.
7. Procédé selon la revendication 6, caractérisé en ce que l'alcool est choisi parmi l'alcool isopropylique, l'éthanol et leur mélange.7. Method according to claim 6, characterized in that the alcohol is chosen from isopropyl alcohol, ethanol and their mixture.
8. Procédé selon l'une des revendications précédentes, caractérisé en ce qu'avant d'effectuer une gravure chimique de la face arrière de la plaquette, on dépose une couche de Borosilicagel et/ou de TiN sur la face avant de ladite plaquette, de manière à y former au moins une couche de protection constituée essentiellement en TiN et/ou en BSG/TiN.8. Method according to one of the preceding claims, characterized in that before performing a chemical etching of the rear face of the wafer, a layer of Borosilicagel and / or TiN is deposited on the front face of said wafer, so as to form therein at least one protective layer consisting essentially of TiN and / or BSG / TiN.
9. Circuit intégré formé sur une plaquette de silicium amincie selon le procédé défini dans les revendications 1 à 8.9. Integrated circuit formed on a thinned silicon wafer according to the process defined in claims 1 to 8.
10. Carte à puce caractérisé' en ce qu'elle comprend un circuit intégré selon la revendication 9. 10. Chip card characterized 'in that it comprises an integrated circuit according to claim 9.
PCT/FR2002/003257 2001-09-27 2002-09-24 Method for thinning a silicon wafer WO2003028077A1 (en)

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