Method for manufacturing semiconductor-on-insulator substrate for radio frequency applications

Abstract

The present invention relates to a method for manufacturing a semiconductor-on-insulator substrate for radio frequency applications, the method comprising the following steps: providing a p-doped semiconductor donor substrate (1); forming a sacrificial layer (13) on the donor substrate; implanting atomic species through the sacrificial layer (13) to form a weakened region (11) in the donor substrate (1) defining a semiconductor thin layer (12) to be transferred; removing the sacrificial layer (13) from the donor substrate (1) after the implantation; providing a semiconductor carrier substrate (2) having a resistivity greater than or equal to 500 Ω.cm; forming an electrical insulating layer (20) on the carrier substrate (2); bonding the donor substrate (1) to the carrier substrate (2), the semiconductor thin layer (12) and the electrical insulating layer (20) at the bonding interface; and separating the donor substrate (1) along the weakened region (11) to transfer the semiconductor thin layer (12) from the donor substrate (1) to the carrier substrate (2).

Description

Technical Field

[0001] The present invention relates to a method of manufacturing a semiconductor-on-insulator substrate for radio frequency applications. Background Art

[0002] Radiofrequency electronic components formed in or on semiconductor substrates are particularly sensitive to attenuation phenomena caused by the properties of said substrate.

[0003] For this purpose, a semiconductor substrate with high resistivity (ie, greater than 500 Ω·cm), in particular a bulk silicon substrate, is usually used.

[0004] Furthermore, FDSOI (abbreviation of the term “fully depleted semiconductor on insulator”) semiconductor-on-insulator substrates appear to be a useful alternative to bulk semiconductor substrates. An FDSOI substrate comprises, in sequence, a carrier substrate, an electrically insulating layer and a thin semiconductor layer in which or on which electronic components can be manufactured. In an FDSOI substrate, the thickness of the semiconductor layer is thin enough to allow complete depletion of the conductive channel of the transistor formed in said layer. Such a layer typically has a thickness of several tens of nanometers. The electrically insulating layer, which typically consists of an oxide, is also commonly referred to as BOX (abbreviation of the term “buried oxide”). The method for manufacturing FDSOI substrates aims to achieve high precision with respect to the thickness of the semiconductor layer and the electrically insulating layer and a high degree of uniformity of these thicknesses within a substrate and from one substrate to another within the same manufacturing batch.

[0005] Therefore, for radio frequency applications, it may be beneficial to form a FDSOI substrate with a carrier substrate composed of a semiconductor material having a high resistivity.

[0006] exist Figures 1A to 1C The method for manufacturing an FDSOI substrate is schematically shown in FIG. This method realizes a layer transfer from a donor substrate to a carrier substrate and is also known as the method name SmartCut TM .

[0007] Reference Figure 1A , a donor substrate 1 , eg made of silicon, covered with an electrically insulating layer 10 , eg made of silicon oxide (SiO 2 ), is provided.

[0008] As schematically indicated by arrows, ion species implantation is performed using, for example, hydrogen and / or helium ions through the electrically insulating layer 10 to form a weakened area 11 in the donor substrate 1. The weakened area 11 defines a thin layer 12 to be transferred.

[0009] Reference Figure 1BThe thus implanted donor substrate 1 is bonded to the carrier substrate 2 via the electrically insulating layer 10, which then performs the function of a bonding layer. Advantageously, the carrier substrate 2 can be a semiconductor substrate with a high resistivity, for example made of silicon. The bonding can be supplemented by a heat treatment.

[0010] Reference Figure 1C , the donor substrate 1 separates along the weakened area 11, resulting in the thin layer 12 being transferred to the carrier substrate 2. The separation can be initiated by a heat treatment.

[0011] A finishing process is then performed on the transferred layer in order to correct implantation-related defects and to smooth the free surface of said layer.

[0012] A semiconductor-on-insulator substrate is thus obtained.

[0013] In the case of FDSOI substrates, the target thickness of the transferred semiconductor layer is between 4 nm and 100 nm, with a relative thickness within each substrate and between the individual substrates manufactured using this method. This uniformity and very low roughness of the transferred layer can be achieved using a finishing method called "batch annealing," a lengthy, high-temperature smoothing method that is advantageously performed in a furnace to process multiple substrates simultaneously. This "batch annealing" is typically carried out at temperatures between 1150°C and 1200°C for a duration of several minutes, typically greater than 15 minutes. This smoothing allows the transferred semiconductor layer to reach a surface roughness level compatible with the subsequent fabrication of transistors.

[0014] However, this approach is detrimental for radio frequency applications, particularly for extremely high frequency applications, namely in the frequency band between 30 GHz and 300 GHz, also known as "millimeter wave (mmWave)".

[0015] In particular, the carrier substrate has a high resistivity and is therefore weakly doped.The carrier substrate is therefore typically much less doped (for example boron doped) than the donor substrate, in other words less doped than the transferred thin layer.

[0016] However, due to this difference in doping levels between the transferred thin layer and the carrier substrate, under the influence of the high thermal budget of the finishing process of the FDSOI substrate and, to a lesser extent, under the influence of the thermal budget of bonding and / or separation, boron atoms diffuse through the electrically insulating layer into the carrier substrate, resulting in a reduction in the resistivity in the surface portion extending from the electrically insulating layer.

[0017] Now, even if this surface portion extends only a few micrometers into the carrier substrate, the drop in resistivity in this region leads to significant electrical losses in the millimeter wave band. Summary of the Invention

[0018] An object of the present invention is to define a method for manufacturing an FDSOI semiconductor-on-insulator substrate suitable for radiofrequency applications, making it possible to maintain a high resistivity of the carrier substrate even in the vicinity of an electrically insulating layer.

[0019] To this end, the present invention provides a method for manufacturing a semiconductor-on-insulator substrate for radio frequency applications, the method comprising the following steps:

[0020] - providing a p-doped semiconductor donor substrate;

[0021] - forming a sacrificial layer on the donor substrate;

[0022] - implanting atomic species through the sacrificial layer to form a weakened region in the donor substrate defining the semiconductor thin layer to be transferred;

[0023] - removing the sacrificial layer from the donor substrate after the implantation;

[0024] - providing a semiconductor carrier substrate having a resistivity greater than or equal to 500 Ω.cm;

[0025] - forming an electrically insulating layer on the carrier substrate;

[0026] - bonding the donor substrate to the carrier substrate, the semiconductor thin layer and the electrically insulating layer at a bonding interface;

[0027] - separating the donor substrate along the weakened area in order to transfer the semiconductor thin layer from the donor substrate to the carrier substrate.

[0028] Therefore, by using a bonding layer formed on a carrier substrate rather than on a donor substrate, and by removing the sacrificial layer from the donor substrate before bonding, despite the difference in doping levels between the transferred thin layer and the carrier substrate, the dopant atoms contained in the bonding layer are prevented from diffusing into the carrier substrate and a reduction in the resistivity of the carrier substrate is prevented.

[0029] Furthermore, the steps of fabricating the FDSOI substrate after bonding remain unchanged, making the method compatible with existing industrial production lines.

[0030] In some embodiments, forming the sacrificial layer includes oxidizing the material of the donor substrate.

[0031] In some embodiments, removing the sacrificial layer comprises wet etching the layer.

[0032] Optionally, the step of removing the sacrificial layer may further comprise removing a surface portion of the thin layer to be transferred from the donor substrate.

[0033] In some embodiments, the donor substrate is a boron-doped donor substrate.

[0034] In some embodiments, forming the electrically insulating layer includes depositing an oxide on the carrier substrate.

[0035] In other embodiments, the electrically insulating layer is formed by oxidizing the carrier substrate.

[0036] In some embodiments, the electrically insulating layer has a thickness between 10 nm and 150 nm.

[0037] In some embodiments, the transferred semiconductor layer has a thickness between 4 nm and 100 nm. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Other features and advantages will become apparent from the following detailed description with reference to the accompanying drawings, in which:

[0039] - Figure 1A is a schematic cross-sectional view of implanting atomic species through an electrically insulating layer disposed on a donor substrate;

[0040] - Figure 1B I have already experienced Figure 1A Schematic cross-sectional view of the bonding of an implanted donor substrate and a carrier substrate;

[0041] - Figure 1C It is a thin layer from Figure 1B Schematic cross-sectional view of transferring a donor substrate to a carrier substrate;

[0042] - Figure 2 is a schematic cross-sectional view of forming a sacrificial layer on a donor substrate;

[0043] - Figure 3 The atomic seeds are injected into the sacrificial layer Figure 2 A schematic cross-sectional view of a donor substrate;

[0044] - Figure 4 is Figure 3 Schematic cross-sectional view of removing the sacrificial layer from the donor substrate after implantation of

[0045] - Figure 5 is a schematic cross-sectional view of forming an electrically insulating layer on a carrier substrate;

[0046] - Figure 6 yes Figure 4 The donor substrate and Figure 5 A schematic cross-sectional view of the bonding of a carrier substrate;

[0047] - Figure 7 is a schematic cross-sectional view of transferring a thin layer from a donor substrate to a carrier substrate to form an FDSOI substrate.

[0048] To make the drawings clearer, the various layers are not necessarily shown to scale.

[0049] Reference numbers that are the same from one drawing to the next indicate elements that are similar or that perform at least the same function. DETAILED DESCRIPTION

[0050] Improve the method of manufacturing FDSOI substrate so that the donor substrate is bonded to the carrier substrate through an electrically insulating layer formed on the carrier substrate instead of the donor substrate. Unlike the donor substrate, the carrier substrate is undoped or at least has a doping ratio less than or equal to 1 E 13 at / cm 3 Thus, the electrically insulating layer does not contain a significant concentration of dopants that could easily diffuse into the carrier substrate during subsequent thermal treatments.

[0051] An electrically insulating layer is still present on the surface of the donor substrate to reduce the effects of direct paths for atomic species during implantation (a phenomenon known by the term "channeling"). However, this layer is sacrificial in that it is removed after implantation and before bonding.

[0052] Figure 2 It is illustrated that this sacrificial layer 13 is formed on a donor substrate 1 .

[0053] The donor substrate is a single crystal semiconductor substrate, for example made of silicon. Typically, the donor substrate is lightly p-doped. For example, the donor substrate contains a concentration less than or equal to 1 E 15 at / cm 3 In particular, donor substrates doped in this way are cheaper and more standardized than undoped donor substrates.

[0054] Sacrificial layer 13 is a conductive layer, in particular made of an oxide of the material of donor substrate 1. Sacrificial layer 13 is advantageously formed by thermal oxidation of donor substrate 1. This thermal oxidation consumes a surface portion of the material of the donor substrate. To prevent channeling, the sacrificial layer advantageously has a thickness between 4 nm and 150 nm, and preferably between 10 nm and 40 nm.

[0055] Reference Figure 3 , ion species are implanted (schematically shown by arrows) into the donor substrate 1 through the sacrificial layer.

[0056] The implanted species typically include hydrogen and / or helium.

[0057] The dose and energy of the implanted seeds are selected to form a weakened area 11 at a predetermined depth in the donor substrate 1 , defining a thin layer 12 to be transferred between the sacrificial layer 13 and the weakened area 11 .

[0058] Reference Figure 3 , and then removing the sacrificial layer 13. Thus, even though the sacrificial layer contains dopants originating from the donor substrate, it is not present during the subsequent bonding.

[0059] The sacrificial layer can be removed, for example, by wet etching. A person skilled in the art will be able to select a suitable etching solution. Such etching leaves the surface of the donor substrate sufficiently smooth and defect-free to allow high-quality subsequent bonding.

[0060] The step of removing the sacrificial layer may optionally be followed by removing a surface portion of the thin layer 12 to be transferred. This surface portion (whose thickness is typically of the order of a few nanometers) may be removed using any suitable means (e.g., by thermal oxidation or by chemical etching) that does not degrade the uniformity of the transferred layer. Less preferably, chemical mechanical polishing may be used.

[0061] Reference Figure 4 , an electrically insulating layer 20 is also formed on the carrier substrate 2 .

[0062] The carrier substrate 2 is a semiconductor substrate, for example made of silicon, which has a high resistivity, for example greater than 500 Ω·cm, preferably greater than or equal to 1000 Ω·cm.

[0063] Particularly advantageously, the carrier substrate is a silicon substrate with a high interstitial oxygen content, i.e., a content greater than 20 old ppma (for the definition of the unit old ppma, see Robert Kurt Graupner's thesis, "A Study of Oxygen Precipitation in Heavily Doped Silicon" (1989), Dissertations and Theses, Paper 1218). Such substrates are often designated by the abbreviation "HiOi." Interstitial oxygen atoms readily precipitate during heat treatment, forming a large number of defects formed by oxygen precipitates, known as "bulk microdefects" (BMDs). These defects block dislocations generated during high-temperature heat treatment, which helps maintain the crystalline quality of the carrier substrate.

[0064] In practice, to use such a HiOi substrate to manufacture an FDSOI substrate, the method includes, before bonding, a step of thermally treating the carrier substrate at a temperature sufficient to induce interstitial oxygen precipitation and form the BMDs. This thermal treatment can typically be performed in a thermal cycle lasting 12 hours and reaching a temperature of the order of 1000°C.

[0065] Furthermore, HiOi substrates typically include a large number of crystal defects called COPs (an abbreviation of the term “crystal origin particles”), which are undesirable in FDSOI substrates. Advantageously, the manufacturing method therefore includes a “depletion” thermal treatment aimed at diffusing oxygen outside the carrier substrate. In practice, this treatment can be performed simultaneously with the thermal treatment for precipitating interstitial oxygen, as long as the surface of the carrier substrate is free, that is to say not oxidized, to allow oxygen to diffuse outside the substrate. In this case, this precipitation / diffusion thermal treatment should be performed before forming the electrically insulating layer on the carrier substrate.

[0066] Alternatively, a person skilled in the art may select a silicon substrate with a low or medium interstitial oxygen content (i.e., a content of less than 10, respectively between 10 and 20 ppma) for the carrier substrate. Such substrates are usually designated by the abbreviations "LowOi" and "MidOi," respectively. In this case, the above-described precipitation and / or diffusion heat treatment is not necessary.

[0067] The electrically insulating layer 20 is advantageously an oxide layer in order to ensure a high-quality bond with the semiconductor material of the donor substrate 1 .

[0068] The electrically insulating layer may be formed by a deposition process, in particular chemical vapor deposition (CVD), or by an oxidative heat treatment of the carrier substrate.

[0069] The thickness of the electrically insulating layer 20 is preferably between 10 nm and 150 nm.

[0070] Reference Figure 5 , the donor substrate 1 is placed in contact with the acceptor substrate 2, with the thin layer 12 to be transferred and the electrically insulating layer 20 at the bonding interface. A molecular adhesive bond then occurs between the oxide of layer 20 and the semiconductor material of layer 12.

[0071] The bonding can be supplemented by methods that prepare the electrically insulating surface, for example using oxygen plasma.

[0072] As described above, compared to the existing method for manufacturing FDSOI substrates, the manufacturing steps after bonding remain unchanged, so this method is compatible with existing industrial production lines and does not affect the physical and electrical characteristics of the product.

[0073] Reference Figure 6 , the donor substrate 1 is separated along the weakened area 11. In a manner known per se, the separation can be caused by applying a mechanical stress in the vicinity of the weakened area, by a heat treatment or by any other suitable means.

[0074] At the end of this separation, the thin layer 12 has been transferred from the donor substrate to the carrier substrate, and an FDSOI structure comprising the carrier substrate 2, the electrically insulating bonding layer 2 and the transferred layer 12 is obtained (see Figure 7 ).

[0075] The structure is then subjected to the finishing treatments usually carried out for FDSOI substrates. These finishing treatments include, in particular, the thermal smoothing ("batch annealing") of the transferred layer as mentioned in the introduction.

[0076] In some embodiments, the smoothing process includes placing a batch of FDSOI structures in a furnace, slowly increasing the temperature from ambient temperature (20°C) to a temperature on the order of 1500°C to 1200°C, and then holding the structures at this temperature for several minutes, preferably greater than 15 minutes.

[0077] Removing the sacrificial layer from the donor substrate before bonding surprisingly makes it possible to reduce the diffusion of dopants into the carrier substrate and the degradation of the high-resistance properties of the resulting structure.

[0078] Without wishing to be bound by this hypothesis, the inventors believe that the formation of a protective oxide layer on the donor substrate may be responsible for the phenomenon of accumulation of dopants at the interface between the donor substrate and said oxide layer, which dopants will diffuse after the donor substrate has been bonded to the carrier substrate. Therefore, removing this sacrificial layer (and optionally the surface portion of the underlying thin layer to be transferred) appears to eliminate or at least reduce this accumulation of dopants.

[0079] Although the thermal budget of this smoothing process is high enough to allow the diffusion of dopants present in the structure, the dopants in the donor substrate are kept sufficiently far away from the carrier substrate by the electrically insulating layer (which does not contain any such dopants) so as not to diffuse into the carrier substrate. As a result, the resistivity of the carrier substrate is not affected, even in its portion close to the bonding interface.

[0080] The resulting FDSOI structure is therefore fully functional for RF applications, particularly in the millimeter-wave band.

Claims

1. A method for manufacturing a semiconductor-on-insulator substrate for radio frequency applications, the method comprising the following steps: - providing a p-doped semiconductor donor substrate (1); - forming a sacrificial layer (13) on the donor substrate; - implanting atomic species through the sacrificial layer (13) to form a weakened region (11) in the donor substrate (1) defining the semiconductor thin layer (12) to be transferred; - removing the sacrificial layer (13) from the donor substrate (1) after the implantation; - providing a semiconductor carrier substrate (2) having a resistivity greater than or equal to 500 Ω·cm; - forming an electrically insulating layer (20) on the carrier substrate (2); - bonding the donor substrate (1) to the carrier substrate (2), the semiconductor thin layer (12) and the electrically insulating layer (20) at a bonding interface; - separating the donor substrate (1) along the weakened region (11) to transfer the semiconductor thin layer (12) from the donor substrate (1) to the carrier substrate (2), wherein the semiconductor-on-insulator substrate is able to maintain the high resistivity of the carrier substrate (2) even in the vicinity of the electrically insulating layer (20) by preventing dopant atoms from diffusing into the carrier substrate (2).

2. The method according to claim 1, wherein The step of forming the sacrificial layer (13) comprises oxidizing the material of the donor substrate (1).

3. The method according to claim 1 , wherein: The step of removing the sacrificial layer (13) comprises wet etching the layer (13).

4. The method according to claim 1 , wherein: The step of removing the sacrificial layer (13) further comprises removing a surface portion of the thin layer (12) to be transferred from the donor substrate.

5. The method according to claim 1 , wherein: The donor substrate (1) is a boron-doped donor substrate.

6. The method according to claim 1 , wherein: The step of forming an electrically insulating layer (20) comprises depositing an oxide on said carrier substrate (2).

7. The method according to claim 1 , wherein: The step of forming the electrically insulating layer (20) comprises oxidizing the carrier substrate (2).

8. The method according to claim 1 , wherein: The electrically insulating layer (20) has a thickness between 10 nm and 150 nm.

9. The method according to claim 1 , wherein: The transferred semiconductor layer (12) has a thickness between 4 nm and 100 nm.

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