Patent Description:
Such a method is known from the patent published under the number <CIT> describing a method for connecting etched silicon wafers by soldering. This method makes it possible to form a sealed cavity by assembling two silicon wafers on which there are deposited either a continuous line of soldering material on the wafer or a continuous line on one of the silicon wafers and an extended metallic layer on the other silicon wafer, and the method then consists of heating so that, the soldering material melting, the connection between the two silicon wafers is achieved.

The English language abstract of <CIT> discloses a method of manufacturing a semiconductor device in which master and slave substrates have functional elements in their central areas. Complementary arranged connection electrodes are provided on the facing surfaces of each substrate. When the substrates are brought together and the functional element regions are connected, a metallic sealing dam of a solder alloy is formed in the periphery of the device hermetically sealing the interior.

<CIT> discloses a semiconductor device face-down bonded with pillars. The device comprises a first semiconductor substrate having an insulating surface on which wiring lines are formed. A second semiconductor substrate has a semiconductor electric/electronic circuit formed on the surface of the second substrate. Wiring pillars made of a conductive material are disposed on the contact terminals provided on the surface of the second semiconductor substrate and face the wiring lines. A plurality of spaced apart support pillars are disposed in an array extending around a space between the first and second substrates and connect the first and second substrates. A reinforcing material covers the surface of the second substrate on the opposite side to the semiconductor surface and extends to the first substrate. The reinforcing material contacts at least some of the supporting pillars and bridges the space between the pillars contacted thereby enabling at least some of the space between the first and second substrates devoid of the reinforcing material. The spacing between the supporting pillars is chosen having regard to the type and viscosity of the reinforcing material.

<CIT> discloses a method of fabricating a microstructure having an internal cavity. The method comprises depositing a first layer or stack of layers in a closed geometric configuration on a first substrate. An indent is formed in the first layer or on the top layer of the stack of layers. A second layer or a second stack of layers is deposited substantially with the substantially closed geometric configuration on a second substrate. The first substrate is aligned on and bonded to the second substrate to create a cavity having the closed geometric configuration. The cavity is pumped by way of the indent which is closed by reflowing the first layer or top layer of the stack of layers. <CIT> discloses a method for preventing a sealing resin from invading the space between a semiconductor device and a substrate by the use of a peripheral ring of solder bumps.

The invention relates to the following considerations. In the state of the art, it is necessary first of all to deposit a layer on at least one silicon wafer. In addition a step of producing the continuous line or lines on one of the two silicon wafers requires an additional manufacturing step by material deposit, evaporation, printing by mask or ion bombardment. This sometimes gives rise to the development of additional tools: masks for making an impression etc. This is recognized as resulting in additional cost.

An object of the present invention is to make it possible to manufacture a sealed cavity by means of a fast and simple method, that is to say one which does not require any additional step or additional tools.

According to a first aspect of the present invention there is provided a method of connecting a surface of a first silicon wafer with a connecting surface of a second silicon wafer in accordance with claim <NUM>.

Such a method advantageously resolves the problem disclosed above by profiting from an obligatory step, the deposition of soldering bumps intended for the electrical contacts, for the manufacture of the sealed cavity.

In an example, the two silicon wafers include functional areas. This embodiment has the advantage of making it possible to combine several functionalities sensitive to the external environment on the same circuit and within the same cavity, thus limiting the number of circuit manufacturing steps and the complexity of the circuit obtained. The purpose of such an embodiment is to make it possible to combine, in little space and with few steps, at least two functions on the same circuit.

In an advantageous embodiment, the method includes a step of filling the cavity with an inert gas. The purpose of this embodiment is to give good insulation of the components present on the silicon wafers inside the cavity, to allow that said components are well isolated from contamination and variations in the external environment. Thus the method can advantageously be implemented in an enclosure filled with an inert atmosphere.

According to a first example, there is provided an integrated circuit made by the method.

More broadly the example relates to any application where it is wished to attach an integrated circuit to another one or to a substrate without connection by wire (by "flip chip"). This is particularly advantageous in applications with discrete components (for example surface wave filters, BAWs (Bulk Acoustic Wave filters), SAWs (Surface Acoustic Wave filters), MEM (Micro Electro-Mechanical) switches etc) in relatively high-frequency applications where the connections give rise to parasitic phenomena. These components can advantageously be etched, one on one silicon wafer, the other on the second silicon wafer and therefore combined, according to the invention, within the same sealed cavity.

Such applications concern in particular telecommunications, TV tuners, circuits intended for wireless, high-rate transmissions etc..

The integrated circuit obtained by the method according to the invention is then advantageously assembled (by bonding or flipping) on a metallic grille and molded in a plastic box. The last step is conventional in the assembly of semiconductor circuits.

According to an example not falling within the claimed invention there is provided an apparatus intended to receive and transmit communication signals, the apparatus comprising at least one antenna, reception and transmission amplifiers, and a unit for processing the received and transmitted signals, said apparatus being characterised in that it includes at least one integrated circuit in accordance with the first example.

An example of said communication apparatus comprises an antenna, a switch advantageously produced by means of an MEM switch, reception and transmission filters, reception and transmission amplifiers as well as a unit for processing the signal received. The filters are here advantageously surface wave filters, for example BAW or SAW filters. The MEMs comprise an electrode which is in suspension in air, this electrode often consisting of a bimetallic strip. This strip must be placed in an sealed cavity since this electrode is sensitive. A cavity according to the invention can advantageously find an application here. In addition, BAW and SAW filters require the presence of a cavity for the resonator to function. The double utility of the invention, which makes it possible to combine the filters with the MEM switch in the same cavity, within the same circuit assembled by the technique of attaching a silicon wafer to another with wireless connection, will be understood here.

The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein:.

The following remarks concern the reference signs. Similar entities are designated by a reference by means of identical letters in all the figures. Several similar entities may appear in a single figure. In this case, a digit or a suffix is added to the reference by letters in order to distinguish similar entities. The digit or suffix may be omitted for reasons of convenience. This applies to the description and to the claims.

The following description is presented to enable a person skilled in the art to produce and make use of the invention. This description is provided in the context of the patent application and its requirements. Various alternatives to the preferred embodiment will be obvious to a person skilled in the art and the generic principles of the invention disclosed here can be applied to other implementations.

<FIG> shows a schematic diagram of the method according to the invention. A first silicon wafer WA1 is subjected to an etching step DES1. Etching here means the implementation of all or some of the known methods of manufacturing functional semiconductors. At the end of this step, the first silicon wafer comprises at least one functional area DA1 which must be protected within a sealed cavity, for example an MEM switch or a surface wave filter. Advantageously, the functional area DA1 is surrounded by a metallic ring produced during previous steps by means of conventional methods (deposition of a metallic layer, filling trenches made in the substrate with metal etc). This optional metallic ring then determines more precisely the surface for connection of the silicon wafer with the second. The functional area also comprises metallic areas for producing electrical contacts. Next, a step of depositing bumps PLTS intended to produce the electrical contacts is performed. These bumps are deposited on the previously mentioned metallic areas. This step is carried out conventionally for the manufacture of circuits produced by turning an integrated circuit over on another without connection by wire. The electrical contacts are then made when the second silicon wafer is turned over on the previous one at the electrical contact bumps situated at the required places. During the step of depositing bumps PLTS, additional bumps not intended for the electrical contacts are then deposited according to the invention on the connection surface surrounding the functional area DA1. If the connecting surface is a metallic ring then the bumps are deposited on this metallic ring. A silicon wafer WAE ready to be assembled in a reflux soldering step RFX on a second silicon wafer WA2 is then obtained.

<FIG> is a plan view of a silicon wafer WAE obtained after this step PLTS of depositing the bumps. It therefore comprises in particular a functional area DA1 and can also comprise various other circuit areas: for example an inductor IND, another functional circuit portion MC etc. The bumps consist of material able to melt at the conventional reflux soldering temperatures used for flip-chip techniques. Thus conventionally the materials used are bumps formed by a tin-lead alloy ("solder bumps"). The bumps deposited in the step PLTS are either electrical contact bumps PLTE or additional bumps PLTC intended to contribute to the formation of a sealed cavity for surrounding the functional area DA1. For example, the size of the tin-lead alloy bumps is generally, with current technologies, around <NUM> microns in diameter. The bumps PLTC intended to form the cavity will then advantageously be placed at a distance of approximately <NUM> microns from each other. These indicative dimensions make it possible to obtain a cavity with a diameter compatible with the elements conventionally implemented on the functional area or areas within a cavity. On the other hand, the bumps PLTS intended for the electrical contacts are conventionally spaced further apart so as to prevent their touching after the reflux soldering step. The dimensions given here are only indicative in the light of current technologies. This does not exclude technology one day making it possible to produce smaller bumps and nevertheless to produce a sealed cavity according to the principle of the invention. Overall, the ratio between the diameter of the bumps and the space between the bumps is between <NUM>/<NUM> and <NUM> according to the diameter of the bumps and the spreading capability of the material of which they consist. Advantageously, it has been seen that the functional area was, optionally, surrounded by a metallic ring MR shown in dotted lines in the figure.

In this case, the bumps PLTC are then deposited on this metallic ring. This makes it possible to ensure better adhesion and reliability of the interface.

Next, referring again to <FIG>, a reflux soldering step RFX is carried out. The purpose of the reflux soldering is to produce the contact with the second silicon wafer WA2. Prior to the soldering step RFX proper, the second silicon wafer WA2 is placed in contact with the soldering bumps. During the soldering step RFX, the assembly is heated to a temperature which makes the alloy melt. For example, with the materials mentioned above, the temperature will be approximately <NUM> C. It is advantageously below <NUM> C in order not to damage the etched functional surfaces. The electrical and adhesion contacts are thus provided between the two silicon wafers WA1 and WA2. The seal is provided by the melting of the bumps, which join together to form a wall which, when the bumps are in accordance with the dimensions mentioned previously, will have a width of approximately <NUM> microns and a height of approximately <NUM> microns. The second silicon wafer WA2 may or not may not have been etched in an optional etching step DES2 and consequently include or not functional areas DA2. Optionally, it advantageously has a ring around the area which will be within the sealed cavity in order to provide good adhesion and reliability of the interface in parallel of the first silicon wafer which advantageously also has a metallic ring around the functional area to be placed within the cavity. This ring can be produced with etching or without etching, by simple deposition.

It should also be noted that here there is described a method in which a first silicon wafer WA1 comprises a functional area DA1, the deposition of the contact bumps being effected on this wafer WA1, but that the method according to the invention can also be implemented using a wafer on which there is no etching but on which there is only carried out the deposition of the soldering bumps, the etched area being included on the second silicon wafer. Although less advantageous, this embodiment can be envisaged according to the invention. <FIG> is therefore only indicative of an intermediate result of a special embodiment.

According to one advantageous embodiment, the reflux soldering step RFX is carried out with filling of the cavity with an inert gas. This filling can advantageously be carried out during the reflux soldering, which is then carried out in air or in an inert gas atmosphere within an enclosure. In this case the soldering, by connecting the bumps together, traps the air or inert gas.

<FIG> depicts an example of an integrated circuit CPC obtainable by the claimed method. This Figure is merely indicative of a particular embodiment in which the two silicon wafers WA1 and WA2 are etched. More precisely, on the first silicon wafer WA1 there is etched a switch MEM in an active area DA1. On the second silicon wafer WA2 there is etched a resonator filter BAW in an active area DA2. The functioning of these areas and their insertion in a functional circuit are guaranteed by the presence of electrical contacts between the two areas and/or with components external to these areas. The contacts between the two areas are provided by virtue of the bumps of soldering material PLTE. These bumps are deposited during a bump deposition step conventionally used in the methods of generating circuits by turning over a silicon wafer on another without connection by wire. Under each of the bumps PLTE intended for the electrical contacts deposited, a metallic area MTDE is present. Advantageously, in contact with these areas MTDE and as required, contacts CCT are provided with components external to the active areas DA1 and DA2. For example, these contacts are hollowed out in the silicon wafer WA1 during the etching step DES prior to the assembly of the two silicon wafers. Advantageously, a metallic material is also deposited under the bumps PLTC intended to form the cavity. This metallic area takes the form of a metallic ring MTDC surrounding the active areas DA1 and DA2.

In a preferred embodiment, referring to <FIG>, a final assembly step STK is carried out. This step consists preferentially of assembling by soldering the circuit CPC consisting of at least the two silicon wafers on a metallic grille MTL and then molding on the assembly in a plastic can PST. The soldering is for example carried out using bumps PLT which may have been deposited on the silicon wafer WA1 during the bump deposition step PLTS previously described. It can be seen, in <FIG>, that the final circuit FC, obtained in an example not falling within the claimed invention, contains not only the circuit CPC but also other circuit portions OPC which may or may not contain insulating cavities. Thus a complete functional circuit is contained on the final circuit FC. An example of a complete functional circuit will now be described.

Claim 1:
A method for connecting a surface of a first silicon wafer (WA1) with a connecting surface of a second silicon wafer (WA2) so as to form an sealed cavity after assembly, at least one of the first and second silicon wafers each including at least one functional area (DA1 or DA2) intended to be within the sealed cavity, characterized by the steps of depositing alloy soldering electrical contact bumps (PLTE) on metallic areas (MTDE) on the connecting surface of the first silicon wafer comprised within the functional area, and, during the step of depositing the electrical contact bumps, furthermore depositing alloy soldering cavity-forming bumps (PLTC) not intended for electrical contacts on a connection surface surrounding the functional area of the first wafer, placing the second silicon wafer in contact with said alloy soldering bumps, and reflux soldering the first silicon wafer on the second silicon wafer, wherein the assembly is heated to a temperature which makes the solder alloy melt, wherein the cavity-forming bumps are separated from one another by an even distance which is sufficiently small to cause the melted cavity forming bumps to join together to form a wall and provide a seal thereby forming the sealed cavity during the reflux soldering, and wherein a ratio between the diameter of
the cavity forming bumps and the distance between adjacent cavity forming bumps is between <NUM>/<NUM> and <NUM> according to the diameter of the bumps and the spreading capability of the alloy constituting the cavity forming bumps,
the electrical contacts between the two functional areas being provided by virtue of the electrical contact bumps (PLTE) .