Structure for radiofrequency applications and process for manufacturing such a structure

Abstract

The invention relates to a structure for radiofrequency applications comprising: a monocrystalline substrate, a polycrystalline silicon layer directly on the monocrystalline substrate, and an active layer on the polycrystalline silicon layer intended to receive radiofrequency components. At least a first portion of the polycrystalline silicon layer extending from an interface of the polycrystalline silicon layer with the monocrystalline substrate layer includes carbon and/or nitrogen atoms located at the grain boundaries of the polycrystalline silicon layer at a concentration of between 2% and 20%. A process for manufacturing such a structure includes, during deposition of at least a first portion of such a polycrystalline silicon layer located at the interface with the monocrystalline substrate, depositing carbon and/or atoms in the at least a first portion.

Claims

1. A structure for radiofrequency applications, comprising: a monocrystalline substrate; a polycrystalline silicon layer directly on the monocrystalline substrate; and an active layer on the polycrystalline silicon layer intended to receive radiofrequency components; wherein at least a first portion of the polycrystalline silicon layer extending from an interface between the polycrystalline silicon layer and the monocrystalline substrate includes atoms selected from the group consisting of carbon and nitrogen located at grain boundaries of the polycrystalline silicon at a concentration of between 2% and 20%.

2. The structure of claim 1, wherein the whole polycrystalline silicon layer contains atoms selected from the group consisting of carbon and nitrogen.

3. The structure of claim 1, wherein a thickness of the polycrystalline silicon layer ranges from 200 nm to 1000 nm.

4. The structure of claim 1, wherein the polycrystalline silicon layer further comprises, on the at least a first portion containing carbon and/or nitrogen atoms, a second portion free from carbon and/or nitrogen atoms in that the concentration in carbon and/or nitrogen in the second portion is less than 0.5%.

5. The structure of claim 4, wherein a thickness of the at least a first portion of the polycrystalline silicon layer ranges from 10 nm to 200 nm.

6. The structure of claim 4, wherein a thickness of the polycrystalline silicon layer ranges from 20 nm to 500 nm.

7. The structure of claim 1, wherein the monocrystalline substrate comprises at least one of monocrystalline silicon having an electrical resistivity greater than 500 Ohm.Math.cm, silicon carbide, and/or germanium.

8. The structure of claim 1, wherein the active layer comprises at least one of a semiconducting material, a dielectric material, a ferroelectric material, and a substructure comprising at least one cavity and at least one suspended element on the at least one cavity.

9. The structure of claim 1, wherein the structure further comprises a dielectric layer on the polycrystalline silicon layer, the active layer being on the dielectric layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the present disclosure will become apparent from the detailed description that follows, based on the accompanying drawings, wherein:

(2) FIG. 1 shows a cross-sectional view of a known semiconductor-on-insulator structure comprising a polysilicon layer;

(3) FIG. 2 shows a cross-sectional view of a structure according to an embodiment of the present disclosure;

(4) FIG. 3 shows a cross-sectional view of a structure according to another embodiment of the present disclosure;

(5) FIGS. 4A to 4D show several steps of the manufacturing process of a structure according to the present disclosure.

DETAILED DESCRIPTION

(6) FIG. 2 illustrates a structure 100 according to a first embodiment of the present disclosure. The structure 100 comprises a monocrystalline substrate 1. The monocrystalline substrate 1 can be a bulk substrate made of a single material or a composite substrate made of a stack of different materials, wherein at least a layer of a monocrystalline material is disposed at a main surface of the substrate.

(7) Advantageously, the monocrystalline substrate 1 is made of monocrystalline silicon having an electrical resistivity greater than 500 Ohm.Math.cm, although other materials could be selected. For example, the following materials could be used to form the monocrystalline substrate 1: silicon, silicon carbide, germanium or a combination of at least two of the materials.

(8) A polycrystalline silicon layer 2 extends directly on the monocrystalline substrate 1. By directly is meant that the polycrystalline silicon is in contact with the material of the monocrystalline substrate 1 at the interface I. In other words, no polycrystalline or amorphous layer is interposed between the monocrystalline substrate 1 and the polycrystalline silicon layer 2.

(9) The polycrystalline silicon layer 2 includes carbon and/or nitrogen, meaning that carbon and/or nitrogen atoms are located at least primarily at the grain boundaries of the polycrystalline silicon. As will be explained in more detail below, the carbon and/or nitrogen atoms are typically introduced during growth of the polysilicon.

(10) The concentration of carbon atoms in the polysilicon layer 2 is in the 2%-20% range, which can be determined using chemical composition characterization techniques such as Auger Electron Spectroscopy or Secondary Ion Mass Spectrometry.

(11) The carbon and/or nitrogen concentration may be uniform throughout the thickness of layer 2 or can vary along the thickness of layer 2.

(12) The thickness of the polycrystalline silicon layer 2 ranges typically from 200 nm to 1000 nm, which is much thinner than in the prior art structure as illustrated in FIG. 1.

(13) A dielectric layer 4, such as an oxide layer, can be present on the polysilicon layer 2. However, this layer 4 is optional.

(14) The structure 100 then comprises, on the polysilicon layer 2 (or, if present, on the dielectric layer 4), an active layer intended to receive radiofrequency components. The radiofrequency components can thus be formed on or in the active layer 3.

(15) The active layer 3 can comprise a semiconducting material, a dielectric material, a ferroelectric material, and/or a substructure comprising at least one cavity and at least one suspended element on the cavity.

(16) FIG. 3 illustrates a structure 100 according to a second embodiment of the invention. Elements designated by the same reference numerals as in FIG. 2 are the same and thus need not being described in detail again.

(17) As compared to the structure of FIG. 2, the structure of FIG. 3 comprises a polysilicon layer 2 that is made of two portions: a first portion 2a, extending from the interface I and a second portion 2b, extending on the first portion 2a. The first portion 2a includes carbon and/or nitrogen. The concentration of carbon and/or nitrogen atoms in the first portion 2a of the polysilicon layer 2 is in the 2%-20% range. The thickness of the first portion 2a ranges from 10 nm to 200 nm.

(18) By contrast, the second portion 2b is substantially free from carbon and/or nitrogen atoms. Assuming that some carbon and/or nitrogen atoms may have contaminated the second portion, for example, during the manufacturing process, the concentration in carbon and/or nitrogen atoms of the second portion 2b is less than 0.5%.

(19) A manufacturing process of the structures illustrated in FIGS. 2 and 3 is described with reference to FIGS. 4A to 4D.

(20) On the one hand, as shown in FIG. 4A, a monocrystalline substrate 1 is provided.

(21) The substrate 1 is introduced in a reactor so as to carry out deposition of the polycrystalline layer. The deposition technique is advantageously Low-Pressure Chemical Vapor Deposition (LPCVD) or plasma-enhanced LPCVD.

(22) To that end, silane (SiH.sub.4), disilane (S.sub.2H.sub.6) or trisilane (Si.sub.3H.sub.8) and methane (CH.sub.4) or methylsilane (SiH.sub.3CH.sub.3) are introduced in the reactor. The temperature in the reactor typically ranges from 500 C. to 900 C.

(23) Concurrently with the introduction of methylsilane or methane, a gas containing carbon and/or nitrogen is introduced into the reactor, so as to introduce the carbon and/or nitrogen atoms into the polysilicon layer being grown. The concentration of the carbon and/or nitrogen-containing gas is comprised between 50 and 300 sccm.

(24) As a result, a first portion 2a of polysilicon including carbon and/or nitrogen is obtained on the monocrystalline substrate 1. The thickness of this first portion 2a is between 10 nm and 200 nm.

(25) According to an embodiment, the growth of polysilicon along with introduction of the carbon and/or nitrogen-containing gas can be continued until a final polysilicon layer having a thickness between 200 nm and 1000 nm is obtained. In such case, the whole polysilicon layer includes carbon and/or nitrogen (FIG. 4B, left).

(26) According to an alternative embodiment, the introduction of the carbon and/or nitrogen-containing gas is stopped after formation of the first portion 2a, and the growth of a second portion 2b of carbon and/or nitrogen-free polysilicon is continued until obtaining a total thickness of the polysilicon layer 2 of from 20 nm to 500 nm (FIG. 4B, right). The advantage of this embodiment is that the quantity of carbon introduced in the reactor is limited and that dirtying of the reactor walls by carbon is thus minimized. Thus, the reactor requires less cleaning than in the previous embodiment.

(27) Then, the monocrystalline substrate 1 covered with the polysilicon layer 2 is removed from the reactor.

(28) On the other hand, as shown in FIG. 4C, a donor substrate 30 comprising a layer 3 intended to form the active layer of the structure is provided. The layer 3 can be delimited in the donor substrate 30, for example, by a weakness zone 31. The weakness zone 31 can be formed by implantation of atomic species or by any other suitable process. These techniques are known per se and do not need to be described in more detail here.

(29) A dielectric layer may be formed on the polysilicon layer and/or on the active layer of the donor substrate. For instance, the dielectric layer can be obtained by an oxidation of the polysilicon layer and/or of the donor substrate. By way of example, such a dielectric layer 4 is illustrated on the active layer 3 of the donor substrate 30 in FIG. 4C. When dielectric layers are formed on both the polysilicon layer and on the donor substrate, both dielectric layers form together, after bonding, a buried dielectric layer.

(30) Then, as shown in FIG. 4D, the monocrystalline substrate 1 and the donor substrate 30 are bonded such that the polycrystalline silicon layer 2 and the active layer 3 (or, if appropriate, the dielectric layer(s) present on layers 2 and/or 3) are at the bonding interface.

(31) A thermal treatment is carried out to reinforce the bond strength. The temperature of this thermal treatment is typically between 100 C. and 1250 C., and its duration generally ranges from 10 seconds to 2 hours.

(32) The thermal budget of such a treatment would be sufficient to induce recrystallization of a polysilicon layer from the interface with the monocrystalline substrate that forms a seed for recrystallization. However, due to the presence of carbon atoms in at least the portion 2a of the polysilicon layer in contact with the monocrystalline substrate 1, the recrystallization kinetics are substantially slowed down since the carbon atoms block the grain boundaries of the polysilicon. Thus, even with a polysilicon region containing carbon and/or nitrogen having a thickness as small as about 100 nm, the polysilicon layer 2 will not recrystallize completely at the end of the thermal treatment.

(33) The active layer 3 is then transferred onto the monocrystalline substrate 1 and polysilicon layer 2 by removing the remainder 32 of the donor substrate 30. The removal can be carried out by fracturing the donor substrate 30 along the weakness zone 31 (SMART CUT process), or by etching and/or grinding the donor substrate so as to leave only the active layer 3.

(34) The resulting structure is shown in FIG. 2 (in the case of a polysilicon layer containing carbon and/or nitrogen on its whole thickness) or in FIG. 3 (in the case where only the first portion of the polysilicon layer includes carbon and/or nitrogen).

(35) In such a structure, the polysilicon layer 2 has a thickness of between 200 nm to 1000 nm (or even from 10 nm to 500 nm in the situation depicted with reference to FIG. 3), which provides, at the surface opposite to the monocrystalline substrate, a greater number of grain boundaries than in a thicker layer 2 as in FIG. 1. As a result, the polysilicon layer traps the electrical charges present at the surface more efficiently than a thicker layer.

(36) In addition, since the polysilicon layer 2 is thinner than layer 2 in FIG. 1, it is also less rough and thus requires less polishing before bonding to the donor substrate. As a result, a smaller loss of material due to polishing is to be taken into account when forming the polysilicon layer.