Integrated electromagnetic shielding device

To isolate at least one electric or electronic element (16, 58), for example an interconnection integrated onto a semiconductor substrate (12), this device comprises at least one isolation means chosen from an isolating layer (84, 86, 90) extending in the substrate and an assembly whose height exceeds that of the element and which comprises, on either side of the element, at least two superposed conductors (60 62 64, 66 68 70), which are integrated into the substrate and extend along the element.

FIELD OF THE INVENTION

The invention relates to an integrated electromagnetic shielding device and, more particularly, to a device for isolating an electric or electronic element integrated onto a semiconductor substrate.

The invention is used, in particular, to isolate electrical signals which propagate in connections formed on semiconductor substrates, or which are generated in integrated circuits or elements of such circuits.

PRIOR ART

Electronic devices are known, for example integrated emitter-receiver circuits, wherein various types of signals coexist. These signals may differ from each other in that their frequencies are different or by the kind of information they convey: some signals may be logic signals while other signals are analog signals.

The coexistence of these different signals adversely affects the functioning of the circuits. By way of example, a digital signal, such as a clock signal, is likely to interfere with an analog signal.

In addition, the integration of inductive elements into these circuits causes electromagnetic coupling between these inductive elements and electric connections, resulting in the propagation of other signals.

Thus, in an integrated receiver, there is a risk that the spectral quality of the frequencies generated by voltage-controlled oscillators is disturbed by the switching operations of clock signals.

In an electronic device, it is thus necessary to effectively isolate the electric connections wherein signals propagate that are likely to disturb the functioning of the device.

In accordance with a known method, this problem is partly overcome in that an interfering connection is inserted between two conductor lines, which conductor lines are subsequently connected to ground or to a DC voltage source.

This solution is diagrammatically shown in cross-section inFIG. 1, which shows a silicon substrate2which is covered with a thin film of silicon dioxide on which the interfering connection6is provided as well as the two conductor lines8and10.

The article “Signal isolation in BiCMOS mixed mode integrated circuits” by K. Joardar, 1995, IEEE, p. 178, also deals with this topic.

The known solution only provides partial shielding: it does not preclude electromagnetic coupling between the interfering connection6and other connections or components, not shown, which are also formed on the substrate2.

In addition, this known solution does not enable the interfering connection6to be properly isolated, in the millimetric and hyperfrequency ranges, with respect to said other connections or components: signals of said frequencies are likely to be exchanged between said other connections or components and the interfering connection6through the agency of the substrate2.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the invention to overcome said drawbacks.

The invention aims at providing a device for isolating at least one electric or electronic element integrated onto a semiconductor substrate, which isolation device is characterized in that it comprises at least one isolation means selected from:an isolating layer which extends in the substrate, andan assembly whose height exceeds that of the element, and which includes, on either side of the element, at least two superposed electric conductors, which are integrated into the substrate and extend along the element.

This electric or electronic element may be an electric connection as well as an electric or electronic component or an integrated circuit or a part of such a circuit, or even a region integrated into the substrate and bounded by the device, with signals in the millimetric, radio frequency or hyperfrequency ranges being capable of propagation in said region, so that the device also forms an integrated propagation medium (which is self-shielding as it were).

In accordance with a particular embodiment of the device in accordance with the invention, the isolating layer is a semiconductor layer which is buried in the substrate, said layer extending below the element and being provided with a doping type that is in opposition to that of the substrate, so as to form a PN junction with said substrate, the device further comprising means of reverse biasing of the PN junction.

In accordance with a further particular embodiment, the device additionally comprises at least one trench, which extends in the substrate, at right angles to the surface of the substrate and along the element, and between this element and another electric or electronic element, the isolating layer filling said trench in order to isolate the elements from each other.

The device in accordance with the invention may comprise two isolating layers, which fill two trenches extending in the substrate, perpendicularly to the surface of the substrate and along the element, on either side of this element.

Each trench may extend below the two superposed conductors.

In accordance with yet another particular embodiment, the device additionally comprises various parallel trenches extending in the substrate, perpendicularly to the surface of the substrate, below the element and transversely to said element, each trench being filled with an isolating layer.

In accordance with a preferred embodiment of the device in accordance with the invention, the superposed electric conductors are electrically interconnected by means of at least one via formed through an electrically insulating material.

In this case, each via may extend from one end to the other end of the superposed conductors or the superposed electric conductors may be interconnected by means of a number of spaced apart vias.

In accordance with yet another particular embodiment, the device in accordance with the invention additionally comprises an electrically conducting layer which extends above the element and connects the two uppermost electric conductors to each other, said conductors being arranged on either side of the element.

In accordance with a further particular embodiment, the device in accordance with the invention comprises an additional isolating layer which extends in the substrate, below the element, and isolates a portion of the substrate, where the element is formed, from the rest of the substrate.

In accordance with yet another particular embodiment, the isolating layer extends in the substrate, below the element, and isolates a portion of the substrate, where the element is formed, from the rest of the substrate, and the isolating means additionally comprises two electric conductors, integrated into the substrate, which are arranged above said portion of the substrate where the element is formed, respectively, on either side of this element, and extend along this element.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

FIG. 2diagrammatically shows an example in accordance with the invention, comprising, for example a P-type, silicon semiconductor substrate12. This substrate is covered with an electrically insulating thin film14, which is made of, for example, silicon dioxide in a thickness below, for example, 10 μm. The element to be isolated is a conductor line16which is formed on the insulating layer14.

It is desired, for example, to isolate the conductor line16from another conductor line18, which extends parallel to line16and is also formed on the insulating layer14.

For this purpose, use is made of an isolating layer which, in the example shown inFIG. 2, is an N-type buried semiconductor layer20obtained by diffusion into the substrate prior to forming the film14. This layer20extends below the conductor line16and is wider than said line16, as is shown inFIG. 2.

A DC voltage source22is provided to reverse bias the PN junction formed by the layer20and the substrate: the −pole of the source22is connected to the substrate12by means of an electrical contact24, and the +pole of this source22is connected to the buried layer20by means of a via26through the isolating layer14.

In this manner, a capacitive isolation of the conductor line16is obtained.

If the substrate12is made of an N-type semiconductor material, then a P-type buried layer20is formed, and use is made of a DC voltage source whose −pole is connected to said P-type buried layer and the +pole is connected to the substrate.

Another example in accordance with the invention is diagrammatically shown inFIG. 3, which also depicts the semiconductor substrate12and the insulating thin film14formed at the surface thereof.FIG. 3also shows the conductor lines16and18which are to be isolated from each other in accordance with the invention.

To achieve this, a deep trench28is formed which extends parallel to the lines16and18and is situated between said lines. This trench is formed across the thickness of the insulating layer14and extends in the substrate12, the depth P of this trench being equal to, for example, 6 μm and the width L being equal to, for example, 2 μm.

After the trench28has been formed, it is filled with an isolating layer29, which may be made from an electrically insulating material or from a conducting or semiconducting material.

Another example in accordance with the invention is diagrammatically shown inFIG. 4, which again comprises the conductor line16. The other conductor line18and possible other elements to be isolated from the conductor line16are not shown in this drawing.

FIG. 4shows two deep trenches30and32extending parallel to each other and to the conductor line16, which trenches extend, respectively, on either side of the conductor line. These two trenches are again formed across the thickness of the insulating layer14and extend in the substrate12.

The depth P and the width L of these trenches may be, for example, the same as mentioned hereinabove.

After these trenches have been formed they are filled with, respectively, isolating layers34and36, which may be made of an electrically insulating material or of a conducting or semiconducting material.

Another example in accordance with the invention is diagrammatically shown inFIG. 5, which again comprises the conductor line16formed on the insulating thin film14covering the substrate12(not shown inFIG. 5).

Several parallel, deep trenches38extend in the substrate, perpendicularly to the surface thereof, and are formed across the thickness of the layer14and below the conductor line16. In addition, as shown in the drawing, these trenches extend perpendicularly to said line16.

Each trench38is filled with an isolating layer40. Besides, each trench has a width L of, for example, 20 μm, which is larger than the width of the conducting track16; the space E between two adjacent trenches38is, for example, 2 μm; the depth of the trenches38is, for example, 6 μm, and the length or thicknesseof the trenches38is, for example, 2 μm.

First, the trenches38are formed in the substrate12, across the thickness of the insulating layer14, and each trench is filled with an isolating layer40, after which the conductor line16is formed.

The trenches38, which are each filled with an isolating layer40, enable the formation of longitudinal currents in the substrate12to be precluded.

In another example in accordance with the invention (also seeFIG. 5), apart from the transverse trenches38, two longitudinal trenches42and44are provided which are filled with isolating layers46and48and are arranged, respectively, on either side of the arrangement of trenches38. These trenches42and44are of the same type as the trenches34and36shown inFIG. 4. They extend along the conductor line16and enable the isolation of said conductor line to be enhanced. To achieve this purpose, it would also be possible to form deep longitudinal trenches in the substrate so as to electrically interconnect the transverse trenches38and thus form an equipotential grid below the conductor line16.

In another example of the invention (seeFIG. 2), two longitudinal trenches50and52are provided which extend parallel to the conducting track16and on either side of the buried layer26. These trenches50and52, which are filled with isolating layers54and56, also enable the isolation of the conductor line16shown inFIG. 2to be increased.

The isolating layers which fill the trenches (irrespective of whether these trenches extend along the element to be isolated or transversely to said element) can be made of a polysilicon having a low resistivity or of an electrically insulating material such as silicon dioxide.

If the trenches extend along the element to be isolated without contacting said element, use can be made of an electrically conducting material, for example polysilicon having a low resistivity, to form the isolating layers used to fill the trenches.

In the example of the invention that is diagrammatically shown inFIG. 6, an element such as the conductor line16formed on the insulating layer14covering the substrate12, can be isolated using a group of at least two (three in the example shown inFIG. 6) superposed electric conductors on either side of the conductor line16. These electric conductors are integrated in the substrate12and extend along the conductor line16.

The height of each group exceeds that of the line16.

It is to be noted that, in accordance with the invention, it is possible to isolate not only a single element, but a plurality of elements, such as two parallel conductor lines, as shown inFIG. 6, namely the conductor line16and another conductor line58formed on the layer14and extending parallel to the line16, which conductor line58is also situated in the region bounded by the two groups of superposed conductors, the height of these groups exceeding that of the line16.

From layer14upwards, the superposed conductors are referenced60,62and64, on one side of the lines16and58to be isolated, and66,68and70on the other side of the lines16and58.

As the Figure shows, the superposed conductors are electrically interconnected by vias formed across an electrically insulating material such as silicon dioxide.

The conductors60and66are formed first on the silicon dioxide layer14. Next, a silicon dioxide layer72which covers the conductors60and66as well as the conductor lines16and58is formed on said silicon dioxide layer, after which two continuous vias74and76(seeFIG. 7) extending on, respectively, the conductors60and66are formed across said layer72.

Subsequently, at the surface of said layer72, the conductors62and68are formed which are in contact with, respectively, the conductors60and66by means of the vias74and76. Next, a silicon dioxide layer77covering the conductors62and68is formed on the layer72, after which two vias78and79are formed across said layer77, said vias being in contact with, respectively, said conductors62and68.

Subsequently, the conductors64and70are formed on the surface of this layer77, said conductors being in contact with, respectively, the conductors62and68through the vias78and79.

Alternatively, instead of utilizing continuous vias to interconnect two adjacent superposed conductors (seeFIG. 7), use can be made of several discrete vias, such as the arrangement of vias74aor78a(seeFIG. 8), which interconnect these two conductors through the isolating layer separating these conductors.

The vias74aand78ashown inFIG. 8are substantially wire-shaped and the spacing between these wires is chosen in dependence upon the frequency of the electric signals to be isolated.

In another example of the invention, two trenches80and82(seeFIG. 6) may be provided, which are of the same type as the trenches30and32shown inFIG. 4, said trenches being arranged below the conductors60and66, respectively. These trenches are filled with isolating layers84and86, which may be made of an electrically insulating material or, conversely, of an electrically conducting material since this material is in contact with the conductors60and66.

In addition to the trenches80and82, or in the absence thereof, it is also possible to provide transverse trenches88of the same type as the trenches38shown inFIG. 5. These trenches88extend below the elements to be isolated, as shown inFIG. 6, and they are filled with isolating layers90which are made of an electrically insulating material.

It is also possible to provide an electrically conducting layer91on the insulating layer77, which electrically conducting layer extends above the elements to be isolated (lines16and58) and interconnects the two uppermost conductors64and70which are arranged, respectively, on either side of these elements such as to form an integrated electromagnetic screen.

Another example of the invention is diagrammatically shown inFIG. 9, wherein the conductor line16to be isolated is shown. On either side of this line16and along said line, there are provided two pairs of electric conductors92–94and96–98. The conductors92and96are formed on the silicon dioxide layer14. Another silicon dioxide layer100is formed on the layer14and covers the conductors92and96as well as the conductor line16.

The conductors94and98are formed on this layer100, and vias102and104, which may be continuous (as in the example shown inFIG. 7) or discontinuous (as in the example shown inFIG. 8) interconnect, respectively, the conductors92and94and the conductors96and98via the layer100.

In addition, an isolating layer106extends below the conductors92and96and below the conductor line16and comes again to the surface of the layer14in the direction of the conductor92and the conductor96in order to thus isolate a region107from the substrate wherein the line16is formed.

An isolating material may be used for the layer106, in which case the method is carried out in the following manner: the silicon substrate initially comprises a buried thin oxide film, whereafter trenches filled with oxide are added so as to be in contact with said oxide layer, thereby forming an isolation well.

To obtain optimum isolation against the effects of high-frequency signals, the additional isolating layer106may be formed by deep diffusion into the substrate12and the isolating layer14: if the substrate is of the P-type (or N-type) use is made of a N-type (respectively P-type) buried layer106.

It is also possible to provide a conducting layer108, which is formed at the surface of the silicon dioxide layer100, above the conductor16, which conducting layer interconnects the rectilinear conductors94and98.

Another example of the invention is diagrammatically shown inFIG. 10. The isolation device shown in saidFIG. 10is simpler than that shown inFIG. 9. In the case ofFIG. 10, use is simply made of the conductors92and96as well as of the isolating layer106which isolates the portion107from the substrate12.

By judiciously choosing the dimensions of a device in accordance with the invention, it is possible to obtain an integrated propagation medium that is compatible with the hyperfrequencies (FIG. 11), and even an integrated waveguide for the millimetric frequencies (FIG. 12).

FIG. 11shows the substrate12, which is also covered with the silicon dioxide layer14. A conducting layer110, for example of polysilicon having a low resistivity, is formed on this layer14.

An isolating layer112, for example of silicon dioxide, covers this conducting layer110, and a rectilinear conductor114is formed on this isolating layer112. This conductor114extends above and along the conducting layer110.

This isolating layer112and this conductor114are covered with another silicon dioxide isolating layer116. Next, another conducting layer118, which is substantially identical to the layer110and extends above said layer110, is formed on the layer obtained by uniting these two isolating layers112and116.

As a result, the conductor114is situated between these two conducting layers110and118and is isolated from these layers, thereby forming a “tri-plate” structure.

Depending on the frequency of the signals to be transmitted by means of this structure, it is possible to provide, or to refrain from providing, vias120and122(preferably two continuous vias) which are arranged, respectively, on either side of the conductor114and interconnect the conducting layers110and118via the isolating layers112and116. The conducting layers110and118, as well as the vias120and122, are advantageously connected to the same voltage reference terminal.

The signals transported by this “tri-plate” structure are thus isolated, in accordance with the invention, from other electric signals transported in other elements on the substrate12outside this “tri-plate” structure.

Yet another example of the invention is diagrammatically shown inFIG. 12, wherein the semiconductor substrate12is also covered by the silicon dioxide layer14. A conducting layer124is formed on this silicon dioxide layer14.

An isolating layer126, for example of silicon dioxide, covers this conducting layer124. Another conducting layer128is formed at the surface of this isolating layer126, above the conducting layer124, and these conducting layers124and128are interconnected, across the isolating layer126, by means of the vias130and132(preferably two continuous vias). The conducting layers124and128as well as the vias130and132are advantageously connected to the same reference voltage terminal.

In this manner a waveguide-type propagation medium is formed.

The electric signals, which are transported by this waveguide (in the isolating region134bounded by the conducting layers124and128and the vias130and132), and the frequencies of which are in the millimetric range, are thus isolated from electric signals that propagate in other elements formed on the substrate12(outside the waveguide).

The conductors, the conductor lines or tracks and the conducting layers mentioned hereinabove are made, for example, of a metal such as aluminum.

The vias are made, for example, of a metal such as tungsten.

In addition, only the example shown inFIG. 2employs a bias source. However, use can be advantageously made of such a source when an isolating layer obtained by N or P-type diffusion is used.

In the examples shown inFIGS. 2 through 10, use is made of an isolating layer provided on the surface of the semiconducting substrate. However, it would be possible to omit this layer in the case of the examples described with respect toFIGS. 11 and 12.

Moreover, in the examples described with respect toFIGS. 2 through 10, one or several conductor tracks are isolated. However, these examples of the invention also enable to isolate for example parts of the circuit which are likely to be affected by external electromagnetic interference.