Abstract:
An electronic chip formed of at least one second elementary chip set in first elementary chip so that the surfaces of the elementary chips are substantially in the same plane wherein the first elementary chip is formed of a heterogeneous substrate including a surface layer above a layer of different doping and defining at least one cavity extending the entire thickness of the second layer and at least one metal interconnection level connecting the at least one second elementary chip to the first elementary chip.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of microelectronics. 
     The present invention more specifically relates to forming composite electronic chips that perform many functions of various types. 
     2. Discussion of the Related Art 
     An electronic system is formed of one or several electronic boards. Each board implements a set of functions, and for this purpose, it includes passive elements, hybrid circuits, and integrated circuits. An integrated circuit is formed of an electronic chip assembled on a support or in a package. 
     Integrating the functionalities of an electronic board in the same integrated circuit especially enables: 
     reducing the cost, since several thousands of chips are collectively formed on a same wafer; 
     increasing speed and reducing power consumption, since unwanted elements between the various functions are minimized within the same integrated circuit; and 
     increasing the reliability for all of these functions, since they are included in a single circuit. 
     However, all functionalities of an electronic board are not always simultaneously integrable in the same integrated circuit. Such is the case when the integration of one function block requires use of a first substrate type and the integration of another function block requires use of a second substrate type. 
     As an example, reference will be made to the case of a GSM portable phone. In such a phone, there is a radiofrequency portion, a block that processes intermediary frequencies, and an analog (baseband) signal processing. Other function blocks are also present. These blocks are, in particular, radio amplifiers, memories, and management and regulation of power supplies. Secondary elements such as a calculator may belong to the system. 
     Technological methods using a massive silicon substrate as in BICMOS processes (U.S. Pat. No. 5,953,600 which is incorporated herein by reference) enable integration on the same electronic chip of: 
     bipolar components operating at several gigahertz; 
     MOS transistors for complex digital and analog circuits; 
     active and passive components that form an electronic memory; and 
     power components. 
     However, these processes that use a bulk silicon substrate do not enable integration, with optimal electric characteristics, of passive components such as capacitors, resistors, and inductances. 
     The degradation of the characteristics of these passive components is caused by the presence of the substrate, which behaves like a ground plane having a high internal resistance. It creates capacitive and electromagnetic couplings over any component located close to it. In the case of radiofrequency applications, eddy currents are generated in the silicon substrate. These capacitive and electromagnetic couplings cannot be precisely calculated. Accordingly, a massive silicon substrate has a non-predictable effect on the passive components and, accordingly, on the integrated function blocks. For operating frequencies of several gigahertz, these effects may be redhibitory. 
     In the previously-mentioned example of a portable phone made with a BICMOS method, the specific cases of the resistance and of the inductance are now considered. 
     In a BICMOS process, resistors are used to bias the quiescent point of bipolar transistors. Such is the case, in particular, for the stages of radiofrequency signal amplification. The D.C. biasing current must be greater than a minimum value. The hazards of the used manufacturing process create an uncertainty about the value of the resistance to be obtained at the end of the manufacturing cycle. It is necessary to increase the average quiescent current to be sure to always be greater than the minimum quiescent current necessary to the proper circuit operation. Any uncertainty, due to the manufacturing process, concerning the value of the resistance to be obtained at the end of the process that causes a useless increase of the power consumption of the circuit. The operating autonomy of a portable phone is an essential feature. The manufacturing method must be able to form precision resistors. 
     These resistors also conduct an A.C. signal having a frequency of several gigahertz. The unwanted elements of the resistor, mainly the stray capacitance with respect to the substrate, must also be as small as possible. 
     The methods using a massive silicon substrate enable forming either single-crystal silicon resistors in the substrate or polysilicon resistors above a thick oxide layer. The value of a silicon resistor depends on the amount of active dopants and on the mobility of the carriers in the silicon layer. These two parameters are better controlled when the resistor is made of single-crystal silicon rather than polysilicon. For an industrial process, the reproducibility of resistors formed in single-crystal silicon is of 5%, while that of polysilicon resistors is 20%. 
     For a method using a massive silicon substrate, the single-crystal silicon resistors are formed in the silicon substrate and isolated therefrom by a junction. This junction has too high a junction capacitance for radiofrequency applications. It is then preferred, to the detriment of the power consumption of the system, to make the resistors in polysilicon despite the lack of reproducibility. 
     It would be desirable, for reducing the imprecision of the resistors, to use single-crystal silicon on insulator resistors. However, this is not practical with current methods of integrated circuit manufacturing on massive silicon. 
     The charge impedance of the transistors operating in radiofrequency ranges is often formed of an inductance. Since the frequencies are high, the value of this inductance is small, which makes it integrable. Practically, one or several spirals defined in a metal level are formed to obtain a value of the inductance of approximately 10 nH. The unwanted elements of this inductance then are: 
     the spiral resistance; 
     the different stray capacitances associated with the inductance, such as: 
     the capacitances between the spirals; 
     the coupling capacitances with the substrate; 
     the electromagnetic coupling with the substrate. 
     Either thick aluminum or copper, which have a low resistance, are used to form such inductances. These two metals, with the thicknesses used, are generally not available in the environment of current manufacturing methods. Further, copper is very contaminating for integrated active components. This makes the implementation of technological processes integrating these components expensive. 
     If the substrate is massive silicon, there exists a strong coupling between the electromagnetic field generated by the inductance and the conductive silicon of the substrate. The penetration depth of the electromagnetic field into the substrate ranges between 50 and 100 μm for a substrate having a resistivity greater than 5 ohm.cm and for frequencies of a few gigahertz. 
     The electromagnetic coupling and the various unwanted elements mentioned hereabove degrade the quality factor of the inductance. This quality factor does not exceed 10 for inductances formed on a massive silicon substrate. It would increase to 40 for inductances formed above an insulator having a minimum thickness of 50 μm by using a metal level of low resistance. This type of inductance cannot be formed in a manufacturing method using a bulk silicon substrate. 
     The integration of all components of a system, such as a portable phone, in a single integrated circuit requires compromises which result in an increase of power consumption and in performance losses. Some passive elements, in particular inductances, must be placed outside of the package containing the chip. 
     Another solution is the forming of a hybrid circuit including several chips, some at least of the chips being other than on bulk silicon to optimize the desired performances. In this case, it should be noted that: 
     the performances obtained with a hybrid circuit are a trade-off between those of the electronic board and those which could have been obtained with a single integrated circuit; 
     the cost is high since the hybridization is performed by handling each chip separately; 
     the performances are average, since the general bulk is high and there remain many unwanted elements; and 
     the reliability of the assembly depends on the mechanical quality of the components and on the quality of the various connections or weldings performed in the hybridization. This reliability is much lower than that of an integrated circuit. 
     An assembly of a plurality of chips is disclosed in the European patent application 0465227 but such an assembly raises problems to have the upper surfaces of the various chips exactly at the same level. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a technique enabling implementation, within a same chip, of electronic functions requiring different substrates for their integration. 
     Another object of the present invention is to combine a method enabling forming passive components optimized for frequencies greater than several gigahertz with a technological process using bulk silicon. 
     A third object of the present invention is to provide a method enabling forming of electronic chips having the following features: 
     a lower cost then that obtained with hybrid techniques; 
     a reliability similar to that of a standard integrated circuit; and 
     maximum electric performances. 
     To achieve these and other objects, the present invention provides an electronic system integrated in a chip formed of a first elementary chip in which is set at least one second elementary chip so that the surfaces of the elementary chips are substantially in a same plane, said at least one second elementary chip being connected to the first elementary chip by at least one metal interconnection level. 
     According to an embodiment of the present invention, the electronic system is adapted to a portable phone and includes, in the second elementary chip, passive radiofrequency elements. 
     The present invention also provides an electronic chip formed of at least one second elementary chip set in a first elementary chip so that the surfaces of the elementary chips are substantially in a same plane, said at least one second elementary chip being connected to the first elementary chip by at least one metal interconnection level. 
     According to an embodiment of the present invention, the surfaces of the elementary chips forming the electronic chip exhibit level differences smaller than 10 micrometers. 
     According to an embodiment of the present invention, the second elementary chip is formed by using a sapphire substrate and the first elementary chip is formed by using a massive silicon substrate. 
     According to an embodiment of the present invention, the first elementary chip is formed in a heterogeneous substrate including a surface layer above a layer of different doping; and at least one cavity is dug into the entire thickness of the surface layer. 
     According to an embodiment of the present invention, the first elementary chip or said at least one second elementary chip includes at least one pad of dimensions at least equal to 10 μm, used as a contact surface for the metal level interconnecting the first elementary chip to said at least one second elementary chip. 
     According to an embodiment of the present invention, one of the elementary chips includes a contaminating material connected to the metal interconnection level via a metal level formed before the contaminating material level. 
     The present invention also provides a method for setting a second elementary chip into a first elementary chip, including the steps of: 
     forming a first elementary chip on a wafer; 
     forming at least one cavity in the first elementary chip; 
     forming and cutting at least one second elementary chip having a height substantially equal to the depth of said cavity; 
     placing said at least one second elementary chip into said at least one cavity; 
     depositing an insulating layer; and 
     forming at least one metal interconnection level between the first elementary chip and said at least one second elementary chip. 
     According to an embodiment of the present invention, the substrate used to form the first elementary chip includes a first heavily-doped layer; the substrate used to form the first elementary chip includes a second layer of different doping; and the etch selectivity or an optical property between the second and the first layer is used to adjust the depth of the cavities dug into the substrate of the first elementary chip. 
     According to an embodiment of the present invention, first interconnection pads are formed on the first elementary chip; second interconnection pads are formed on said at least one second elementary chip; and a metal level defined by photolithography connects first and second interconnection pads. 
     According to an embodiment of the present invention, the interconnection pads of one of the elementary chips have dimensions greater than the possible lateral misalignment resulting from the setting of the second elementary chip into the first one. 
     According to an embodiment of the present invention, a second elementary chip is formed by using a silicon-on-sapphire technology and it includes inductances made in copper. 
     According to an embodiment of the present invention, a second elementary chip is formed by using a silicon-on-insulator technique and it includes resistors formed in single-crystal silicon. 
     According to an embodiment of the present invention, after creation of the cavities, a barrier layer is deposited on the wafer to cover the sides of the cavity. 
     According to an embodiment of the present invention, the first elementary chip is formed on a massive silicon substrate; and said at least one second elementary chip includes passive elements used in radiofrequency. 
     The foregoing objects, features, and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a hybrid assembly of two electronic chips according to prior art; 
     FIG. 2 is a cross-section view illustrating an assembly of electronic chips; 
     FIGS. 3 to  6  show, according to a specific embodiment of the present invention, the manufacturing of a wafer including chips using two types of substrate; and. 
     FIG. 7 shows, according to an embodiment of the present invention, a chip formed in silicon-on-sapphire technology and including one resistor and one inductance. 
    
    
     DETAILED DESCRIPTION 
     In the different drawings, homologous elements are designated with same references. Further, as usual in the representation of integrated circuits, the various drawings are not to scale. 
     FIG. 1 shows a hybrid circuit according to prior art. A hybrid circuit is a first approach for the integration of the functions of an electronic board. This approach enables associating, in the same unit, several chips formed on different types of substrates, as well as non-integrated passive elements. For example, FIG. 1 shows a hybrid assembly of two chips D 1  and D 2 . The two chips are glued on a support S made of a ceramic material. Support S supports conductive metal tracks and passive elements such as capacitors, resistors, or inductances. Welded gold wires form connections between pads P 1  of support S and pads P 2  of the chips. It is also possible to directly interconnect the chips due to pads P 3  and P 4  respectively formed on chips D 1  and D 2 . When necessary, conductive wires a few millimeters long create inductances L. As an example, such an inductance is shown on FIG. 1 between a pad P 5  of chip D 2  and a pad P 6  of support S. Support S and the elements thus associated are then altogether used as an independent unit. This unit enables assembly of electronic chips formed on different substrates. These substrates are, for example, bulk silicon, gallium arsenide (AsGa), silicon on insulator (SOI). 
     As illustrated in FIG. 2, two chips D 1  and D 2  are integrated in a single chip D. Chips D 1  are formed on a substrate of a first type. Within each chip D 1 , by using photolithography methods, a cavity is formed. Chips D 2  are formed on a substrate of a second type. Chips D 2  are cut up. Chips D 2  have the size of the cavities formed in chips D 1 . In particular, the thickness of chips D 2  is equal to the cavity depth. This can be obtained either by using a substrate of adequate thickness for the manufacturing of chips D 2 , or by grinding, at the end of the manufacturing process, the wafers supporting chips D 2  to adjust them to the desired thickness. Chips D 2  are then placed in the cavities formed for this purpose in chips D 1 . The resulting wafer thus includes the first and the second type of substrate. Then, the interconnections of chips D 1  with chips D 2  are formed by standard methods of conductive layer deposition and photolithography. A problem of such a structure is that it is not possible to assert with a high precision that the upper surfaces of chips D 1  and D 2  are exactly at the same level. 
     FIG. 3 shows, according to an embodiment of the present invention, a silicon wafer W 1  from which chips D 1  will be formed. This silicon wafer has a standard 725-μm thickness for a wafer with a 200-mm diameter. This silicon wafer has, across its thickness, two layers. First layer  20  is formed, for example, of P+-type silicon, heavily-doped with boron and has a resistivity lower than 0.1 ohm.cm. Second layer  21  is obtained by P-type silicon epitaxy. The thickness of this layer is close to the thickness of chips D 2 , its resistivity is 20 ohms.cm. Active and passive components are formed in upper portion  22  of layer  21 . Several aluminum interconnection levels are then formed at the wafer surface, in a layer  23 . The last metal level  24  includes pads P 2  having, for example, a 80×80-μm 2  surface area, and pads P 3  having, for example, a 10×10-μm 2  surface area. At least one area A free from any component and having an upper surface area of 500×500 μm 2  is saved in each chip D 1  thus formed. 
     FIG. 4 illustrates the next step of the disclosed embodiment of the present invention. A cavity  30  is dug, through successive layers  23 ,  22 , and  21  in each chip D 1  of silicon wafer W 1 . This cavity is centered on area A and is located inside of said area. This cavity is etched by using standard photolithography means and plasmas based on halogenated compounds. According to the compositions of the gases and to the etch conditions, it is possible to start the etching anisotropically to keep the initial mask dimension. A change in the etch method optimizes the etch speed to quickly arrive close to P+-type layer  20 . A last change in the etch method enables stopping the etching by using the etch selectivity between P silicon and P+ silicon. The depth of cavities  30  is determined by the reproducible thickness of layer  21 . This mode of forming cavities  30  enables adjusting, with precision, the surface dimensions and the depth of these cavities to the dimensions of chips D 2 . After removal of the mask and cleaning, a barrier layer  31  is deposited. This layer is, for example, silicon nitride of a 0.1-μm thickness. 
     As illustrated in FIG. 5, a chip D 2  is inserted into an opening A of a chip D 1 , while the silicon wafer has not yet been cut up in individual chips. According to an embodiment of the present invention, an insulating layer  40  is deposited. It is formed, for example, by three depositions. The first deposition is an oxide deposited by chemical vapor deposition (CVD). This first deposition of, for example, a thickness of 1 μm ensures the mechanical hold of chips D 2 . A second spin-on glass (SOG) deposition generates a planar surface and fills the residual cavities. This second deposition has, for example, a thickness of 5 micrometers. A third oxide or nitride deposition (of CVD type) protects the surface. This third deposition has, for example, a 0.2-μm thickness. Layer  31  insulates the circuits of wafer W 1  from the possible pollutants brought by chip D 2 . A small step, smaller than from 1 μm to 10 μm, may remain between the surface of electronic chips D 2  and the surface of wafer W 1 . 
     FIG. 6 shows wafer W 1  after forming of connections. Contact holes  50  emerge on pads P 3  and P 4 . The size of pads P 3  and P 4  and the sizes of the contact holes take into account the imprecision of the alignment of the surfaces of the two chips and the imprecision of the position of chip D 2  in cavity  30 . For example, pads having a 10-μm side will be used in a submicronic manufacturing method. An aluminum metal level  52  is deposited and etched. An oxide and nitride protection layer  53  is deposited over the entire wafer. This protection layer is etched above interconnection pads P 2 . 
     After cutting up wafer W 1 , chips D are obtained, each of which is formed of one or several chips D 2  set into a chip D 1 . Interconnection pads P 2  are the conventional pads of a manufacturing method implemented by using bulk silicon, although chip D includes a copper level and an SOS substrate. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. 
     Any type of system with integrable elementary units is within the scope of the present invention. The units may have electronic functions such as digital, analog, storage signal processing, sensor, detector functions. Mechanical functions are also well adapted to the present invention; accelerometers, pressure sensors are examples. Optical functions, such as laser generation and detection, may, once integrated, make use of the present invention. Finally, thermal functions such as those implemented in a Pelletier cell may also make use of the present invention. 
     The types of substrate simultaneously usable in the same chip only depend on the functions to be implemented. The following material can in particular be used as substrates: silicon, gallium arsenide, germanium, sapphire, zirconia, silicon carbide, ferromagnetic materials. The physical properties of the original substrate given as an example hereabove (thickness, wafer diameter) are by way of example only and do not limit the present invention. 
     The number of cavities  30  is not limited. These cavities may be created at various times in the manufacturing process of first chip D 1 . The cavities may receive chips of type D 2  formed with different substrates. The cavities may receive chips of type D 2  including different circuits. 
     In an embodiment of the present invention, it is provided to use a silicon-on-sapphire substrate. All devices and interconnections available with this type of substrate are directly usable in the case of the described embodiment. Thus, all types of transistors may be formed on chip D 2 : MOS, JFET, bipolar. 
     Any passive element is capable of being used in the present invention: metallic resistors, capacitors using contaminating conductors or dielectric, inductances including a core of a metallic material. 
     In an embodiment of the present invention, a massive silicon substrate is used as the first substrate type and a silicon-on-sapphire substrate (SOS) is used as the second substrate type. A manufacturing method using an SOS substrate enables forming components optimized for operation at radiofrequencies. FIG. 7 shows a specific example of a chip D 2 . This chip uses a single-crystal sapphire  1  (crystallized Al 2 O 3 ). The thickness of this single-crystal ranges between 50 and 300 μm. 
     A single-crystal silicon resistor R is shown on the left-hand portion of FIG. 7. A copper inductance L is shown on the right-hand portion of FIG.  7 . The two components are seen in cross-section. A thin single-crystal silicon layer is formed by epitaxy on the single-crystal sapphire  1 . The thickness of this layer approximately is 0.5 μm. By using photolithography steps and ion implantation techniques, silicon strips including at each end heavily-doped contact heads  3  separated by a lightly-doped bulk  2  of the same conductivity type are formed. The value of the resistance obtained between the two contact heads depends on the conditions used for the implantations. For example, for area  2 , phosphorus is implanted to obtain a concentration of 10 19  at./cm 3  and, for area  3 , arsenic is implanted to obtain a concentration of 10 21  at./cm 3 . 
     A first insulating layer  4  in which contact openings  5  are etched is deposited. A first metal layer  6  in an aluminum alloy is deposited and etched. This first interconnection level has three functions: 
     connecting devices formed in the single-crystal silicon; 
     forming pads P 4  with a surface of approximately 10×10 μm2, to then connect chips D 2  to chips D 1 ; and 
     connecting elements which will be formed in the next metal level. 
     A second insulating layer  7  is then deposited. In the continuation of the process, contact openings  8  are defined through insulator  7 . A copper metal level  9 , deposited afterwards, is spiral-shaped to favor the self inductance of the deposited conductor. The connections of the inductance thus formed with the other components of chip D 2  are performed through contact openings  8  by means of metal level  6 . 
     A third insulating layer  10  is deposited. It enables protection of chips D 2  and in particular of copper  9 , which will never again be exposed in the manufacturing process. 
     The embodiment described in FIG. 7 only includes two interconnection levels, the second being contaminating and non-usable for connection with the other elements of chip D. It is possible to use any number of interconnection levels, each of these levels having specific features to be specifically processed. 
     The dimensions mentioned for cavities  30 , pads P 2 , P 3 , P 4 , have been given as an example only and depend on the techniques used. 
     The control of the depth of cavities  30  is important since the possibility of forming, at the end of the process, a metal interconnection level defined by photolithography partly depends on it. The described embodiment provides a heterogeneous original substrate including two distinct layers  20  and  21 . The selectivity of a plasma etching of layer  21  with respect to layer  20  enables stopping the etching of cavities  30  at the interface of layers  20  and  21 . 
     As described in relation with FIG. 5, the mechanical anchoring chip D 2  in chip D 1  may be obtained by means of a glass deposition. Any other welding or gluing method is possible and depends on the materials used. In FIG. 5, the surface of wafer W 1  is made planar by spun-on glass (SOG) deposition. Other mineral or organic depositions provide the same result. This deposition should be compatible with the subsequent interconnection levels. 
     A single interconnection level has been described to connect the second chips to the first chip (FIG.  6 ). The number and the types of metal levels used after setting of chips D 2  is not limited. 
     Finally, chips D 2  may be, like chips D 1 , formed on a bulk silicon substrate and then have the advantage of enabling selection of the optimized dopings, performing specific thermal cycles and/or being isolated from the components of chips D 1 . 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.