Abstract:
A method for forming through vias connecting the front surface to the rear surface of a semiconductor substrate, including the steps of: forming openings in the substrate, thermally oxidizing walls of the openings, filling the openings with a sacrificial material, forming electronic components in the substrate, etching the sacrificial material, filling the openings with a metal, and etching the rear surface of the substrate all the way to the bottom of the openings.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a method for forming an insulated electric connection between the front surface and the rear surface of a substrate. This electric connection is currently designated in the art as a through silicon via or TSV. 
     2. Description of the Related Art 
     In some electronic devices, chips are stacked. This increases the functions carried out by this type of device, without increasing the occupied surface area. A large selection of technological platforms is available for this purpose. 
     In such devices, to connect the different chips, it can be provided to use known techniques of wire bonding or vias crossing the semiconductor substrate of the chips. 
     In addition to being able to be collectively formed in silicon wafers, vias have the advantage of improving the electric performance of devices. 
     A known via forming method is the following. Once the electronic components have been made in the semiconductor substrate (this substrate being possibly thinned down), through vias are formed. Openings are created in the rear surface of the substrate by dry etch or chemical etch. The walls of each of the openings are then insulated by deposition of a silicon oxide layer by a method of chemical vapor deposition (CVD) type. The openings are filled with copper, by the use of an electrolytic growth method. A through via is thus obtained. Since the vias are insulated after the electronic components have been formed, the silicon oxide deposition method is submitted to thermal stress. The insulator may not be formed at a temperature exceeding 200° C. In such conditions, the silicon oxide can only be deposited by a low-temperature CVD type method. The obtained insulator is of poor quality and is not conformal. It has a degraded electric performance. 
     Further, when the vias are formed from the rear surface, the alignment references with respect to the front surface are lost. To provide a sufficient insulation between vias, the distance separating them is increased. The via integration density is thus decreased. 
     To overcome these disadvantages, it may be provided to form the through vias before forming the electronic components in the substrate. The substrate is first etched to form a plurality of openings. A step of thermal oxidation of the opening walls is then carried out. This step, performed at approximately 1,000° C., provides an insulator of good conformality. A filling with polysilicon is then performed. However, this material is very resistive. A filling with a metal such as aluminum, tungsten, or copper cannot be performed since these metals tend to diffuse into the substrate through the insulator during the anneal steps to form the components. 
     BRIEF SUMMARY 
     An embodiment of the present disclosure provides a method for forming through vias avoiding at least some of the disadvantages of usual via forming methods. 
     Another embodiment of the present disclosure is to provide a method for forming through vias capable of being used in high-voltage and high-insulation applications. 
     Another embodiment of the present disclosure is to provide a method for forming through vias enabling to increase the density of vias in the semiconductor substrate, as well as the accuracy of their positioning. 
     Thus, an embodiment of the present disclosure provides a method for forming through vias connecting the front surface to the rear surface of a semiconductor substrate, comprising the steps of: 
     a) forming openings in the substrate, 
     b) thermally oxidizing walls of the openings, 
     c) filling the openings with a sacrificial material, 
     d) forming electronic components in the substrate, 
     e) etching the sacrificial material, 
     f) filling the openings with a metal, and 
     g) etching the rear surface of the substrate all the way to the bottom of the openings. 
     According to an embodiment of the present disclosure, steps a) to g) occur in the indicated order. 
     According to an embodiment of the present disclosure, step g) occurs between steps d) and e). 
     According to an embodiment of the present disclosure, the semiconductor material is silicon. 
     According to an embodiment of the present disclosure, the sacrificial material is silicon-germanium or polysilicon. 
     According to an embodiment of the present disclosure, the etching of the sacrificial material is performed by photolithography. 
     According to an embodiment of the present disclosure, the sacrificial material is silicon oxide. 
     According to an embodiment of the present disclosure, the metal is selected from the group comprised of copper, tungsten, and aluminum. 
     The foregoing features and advantages of the present disclosure will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIGS. 1A to 1E  are cross-section views illustrating steps of forming through vias, according to an embodiment of the present disclosure; and 
         FIGS. 2C and 2D  are cross-section views illustrating steps of forming through vias, according to another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale. 
       FIGS. 1A to 1E  are simplified cross-section views illustrating steps of forming through vias in a semiconductor substrate  1 . 
     In  FIG. 1A , openings  2  have been formed in substrate  1  by dry or wet etch of the semiconductor material, which advantageously is silicon. A thermal oxidation of the opening walls has then been carried out. A silicon oxide layer  4  having a thickness ranging between 0.1 and 1.5 μm, preferably between 0.2 and 1 μm, has been obtained. Openings  2  have then been filled with a sacrificial material  6 . Sacrificial material  6  for example is silicon-germanium or polysilicon. 
     In  FIG. 1B , MOS transistors T 1  and T 2  have been formed in substrate  1 . Each of the transistors, very schematically shown, comprises a gate  7  and source and drain areas arranged on either side of the gate. A full-wafer insulating layer  8  has then been deposited. As an example, the insulator  8  may be oxide obtained from TEOS (TetraEthyl OrthoSilicate). Openings  9  have been etched by photolithography in layer  8  above the openings  2  filled with sacrificial material  6 , and openings  10  have been etched above the regions of the components where a contact is desired to be made (gates  7  in the shown example). 
     In  FIG. 1C , sacrificial material  6  has been removed by selective etching. As an example, it may be provided to use a wet etch method. A first metal layer has been deposited in openings  2 ,  9 , and  10  to form vias  12  of the first metal level. A second metal layer has been deposited to form metal lines  13  in contact with the vias  12 . For clarity, the metal layer has been hatched in the different drawings. As an example, the metal may be aluminum, tungsten, or copper. 
     In  FIG. 1D , a second metal level has been formed. Vias  14  and lines  15  of the second metal level are of the same metal as the first metal level. As a variation, the metal may be different, that is, aluminum, tungsten, or copper, according to the needs of the technology. Vias  14  which advantageously are of same dimension have thus been formed. 
     In  FIG. 1E , a chemical-mechanical polishing of the rear surface  20  of substrate  1  has been performed to define an access to vias  12 ,  14 . So-called through vias  12 ,  14  connect the front surface  22  to the rear surface  20  of substrate  1 . Vias  12 ,  14  may have a height ranging between 20 and 350 μm. The diameter of the vias may range between 1 and 30 μm. This type of via  12 ,  14  enables the electrical connection of stacked chips. It also enables the transfer of the connection to the rear surface  20 , while said connection is usually taken on the front surface  22 . 
     If the device obtained in  FIG. 1D  has a thickness smaller than approximately 180 μm, it is provided to attach it to a handle before the polishing step. The handling of the substrate is thus eased. 
     For simplification, it has been chosen to only shown two metal levels. In practice, there will be more than two metal levels. 
     According to an advantage of the above-described method, this method enables forming vias with a metal having satisfactory conduction properties. 
     According to another advantage of the above-described method, the vias have a significant insulation quality since oxide  4  is a thermal oxide of good quality. The insulation quality enables use of the chip for high-voltage, high-insulation, or radio frequency applications. 
     According to another advantage of the above-described method, since openings  2  have been formed before the electronic components, there is no alignment problem at the time when the vias are opened at the rear surface at the end of the method. The vias can thus be placed closer to one another, which enables an increase in the integration density in the substrate  1 . 
       FIGS. 2C and 2D  are simplified cross-section views illustrating steps of forming vias and of their opening at the rear surface  20 . 
     The steps preceding those shown in  FIG. 2C  are identical to those illustrated in  FIGS. 1A and 1B . 
     In  FIG. 2C , while the electronic components have been formed in the silicon and no high-temperature anneal step is provided, a first metal layer has been deposited in openings  9  and  10  to form vias  18 . After this, the second metal layer has been deposited in to form the lines  13  of the first metal level. 
     In  FIG. 2D , a second metal level has been formed. A chemical-mechanical polishing of the rear surface  20  of substrate  1  has then been performed, to have access to openings  2  filled with sacrificial material  6 . 
       FIG. 2D  corresponds to  FIG. 1E . Sacrificial material  6  has been etched selectively over insulating layer  4  and metal  18 , from the rear surface  20 . In the openings  2  thus formed, a metal layer  24  identical to the metal layer in via  18  deposited to form the metal levels has been formed. As a variation, it may be decided to use a different metal to fill openings  2 . 
     As a variation, it may be envisaged to use the thermal oxide forming layer  4  as a sacrificial material. In this embodiment, openings  2  are integrally filled with the thermal oxide. Before etching the sacrificial material, a photolithography step is performed. It is ascertained to keep an insulating layer on the opening walls. The thickness of this insulating layer may range between 0.2 and 5 μm, preferably, between 0.2 and 1 μm. 
     Specific embodiments of the present disclosure have been described. Different variations and modifications will occur to those skilled in the art. In particular, the semiconductor substrate may be of SOI type (Silicon On Insulator). Further, for clarity, the deposition of a barrier layer, Ti—TiN, on insulating layer  4 , has not been shown. The titanium layer is capable of bonding to insulating layer  4 , while the titanium nitride is capable of forming an interface layer between insulating layer  4  and the metal level of the via. It may further be provided to use a barrier layer based on tantalum Ta—TaN. The deposition of the barrier layer for example is a physical vapor deposition (PVD). As a variation, it may be provided to simultaneously form the vias and the lines of a same metal level. 
     Various embodiments with different variations have been described hereabove. It should be noted that those skilled in the art may combine various elements of these various embodiments and variations. 
     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 disclosure. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.