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: forming a donor substrate (1) by epitaxially growing an undoped semiconductor layer (101) on a p-doped semiconductor seed crystal substrate (100); forming an electrical insulating layer (10) on the undoped epitaxial semiconductor layer (101); implanting ion seeds through the electrical insulating layer (10) to form a weakened region (11) in the undoped epitaxial semiconductor layer (101) defining a semiconductor thin layer (12) to be transferred; providing a semiconductor carrier substrate (2) having a resistivity greater than or equal to 500 Ω.cm; bonding the donor substrate (1) to the carrier substrate (2) via the electrical insulating layer (10); 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] In addition, FDSOI (abbreviation of the term "fully depleted semiconductor on insulator") semiconductor-on-insulator substrates appear to be a useful alternative to 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 the layer. Such a layer typically has a thickness of several tens of nanometers. The electrically insulating layer, which is usually composed of an oxide, is also commonly referred to as BOX (abbreviation of the term "buried oxide"). The process 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 within the same manufacturing batch and from one substrate to another.

[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 implements a layer transfer from a donor substrate to a carrier substrate, also known as the process name Smart Cut TM .

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

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

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

[0010] refer to Figure 1C , the donor substrate 1 is separated along the weakened area 11 , resulting in the thin layer 12 being transferred to the carrier substrate 2 .

[0011] A finishing treatment 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 variations within each substrate and between the individual substrates manufactured using this method relative to the target value. This uniformity and very low roughness of the transferred layer can be achieved using a finishing process called "batch annealing," a lengthy, high-temperature smoothing process 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 process is detrimental to radio frequency applications, particularly those in the extremely high frequency band between 30 GHz and 300 GHz, also known as “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 (eg 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 of the surface portion extending from the electrically insulating layer.

[0017] Now, even if this surface portion extends only a few micrometers deep into the carrier substrate, the drop in resistivity in this area leads to significant electrical losses for the mmWave waves. 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 proposes a method for manufacturing a semiconductor-on-insulator substrate for radio frequency applications, the method comprising the following steps:

[0020] - forming a donor substrate by epitaxially growing an undoped semiconductor layer on a p-doped semiconductor seed substrate;

[0021] - forming an electrical insulating layer on the undoped epitaxial semiconductor layer;

[0022] - implanting ionic species through the electrically insulating layer to form a weakened zone in the undoped epitaxial semiconductor layer defining the semiconductor thin layer to be transferred;

[0023] - providing a semiconductor carrier substrate having a resistivity greater than or equal to 500 Ω.cm;

[0024] - bonding the donor substrate to the carrier substrate via the electrically insulating layer;

[0025] - separating the donor substrate along the weakened zone to transfer the semiconductor thin layer from the donor substrate to the carrier substrate.

[0026] By this method, the dopants of the seed substrate are kept sufficiently far away from the bonding interface by the epitaxial layer and the electrically insulating layer (which does not contain any such dopants) so that they cannot 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.

[0027] In some embodiments, the thickness of the undoped epitaxial semiconductor layer is between 10 nm and 1000 nm.

[0028] In some embodiments, the seed substrate is boron doped.

[0029] In some embodiments, the step of forming the electrically insulating layer includes thermally oxidizing the material of the undoped epitaxial semiconductor layer.

[0030] In some embodiments, the step of forming a donor substrate includes forming an intermediate layer made of a material different from the material of the epitaxial semiconductor layer between the seed substrate and the undoped epitaxial semiconductor layer, and the material of the intermediate layer is selected to allow the undoped epitaxial layer to be selectively etched relative to the intermediate layer.

[0031] In some embodiments, the material of the undoped epitaxial layer is silicon, and the material of the intermediate layer is silicon germanium with a germanium content less than or equal to 30%.

[0032] In some embodiments, the method includes the following steps: after separation, selectively etching the remaining portion of the undoped epitaxial semiconductor layer relative to the intermediate layer, and then selectively etching the intermediate layer relative to the seed substrate, and forming a new donor substrate by sequentially forming a new intermediate layer and a new undoped epitaxial layer on the seed substrate.

[0033] In some embodiments, the thickness of the electrically insulating layer is between 10 nm and 150 nm.

[0034] In some embodiments, the transferred semiconductor layer has a thickness between 4 nm and 300 nm. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Other features and advantages will become apparent from the following detailed description with reference to the accompanying drawings, in which:

[0036] - Figure 1A is a schematic cross-sectional view of implanting atomic species through an electrically insulating layer disposed on a donor substrate;

[0037] - Figure 1B Has already experienced Figure 1A Schematic cross-sectional view of the bonding of an implanted donor substrate and a carrier substrate;

[0038] - Figure 1C is the process of transferring a thin layer from a donor substrate to Figure 1B A schematic cross-sectional view of a carrier substrate;

[0039] - Figure 2 is a schematic cross-sectional view of forming a donor substrate by growing an undoped epitaxial layer on a doped seed substrate;

[0040] - Figure 3 is Figure 2 A schematic cross-sectional view of forming an electrical insulating layer on an epitaxial layer;

[0041] - Figure 4 yes Figure 2 and Figure 3 Schematic cross-sectional view of an alternative embodiment of the present invention, comprising growing an intermediate layer between a seed substrate and an undoped layer;

[0042] - Figure 5 Ion species are injected through the electrically insulating layer Figure 4 A schematic cross-sectional view of a donor substrate;

[0043] - Figure 6 yes Figure 5 A schematic cross-sectional view of bonding a donor substrate with a carrier substrate having high resistivity;

[0044] - Figure 7 is a schematic cross-sectional view of transferring a thin layer from a donor substrate to a carrier substrate;

[0045] - Figure 8 Is recycled by Figure 7 Schematic cross-sectional view of a first step of transferring the remaining portion of the resulting donor substrate;

[0046] - Figure 9 is a schematic cross-sectional view of a second step of recovering the remaining portion of the donor substrate;

[0047] - Figure 10 is a schematic cross-sectional view of the growth of a new intermediate layer on a seed substrate due to recycling;

[0048] - Figure 11 is Figure 10 Schematic cross-sectional view of epitaxial growth of a new undoped semiconductor layer on an intermediate layer.

[0049] To make the drawings clearer, the various layers are not necessarily shown to scale.

[0050] 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

[0051] The manufacturing method is achieved by Smart Cut TM An undoped semiconductor epitaxial layer is formed on a p-doped seed substrate conventionally used in the process to prevent dopants from diffusing from a donor substrate into a carrier substrate. The assembly consisting of the seed substrate and the epitaxial layer forms a donor substrate, which is intended to receive ion seed implantation and bond to a carrier substrate.

[0052] The thickness of the epitaxial layer is greater than the thickness of the semiconductor layer to be transferred.

[0053] Thus, during the method for manufacturing an FDSOI substrate, the seed substrate comprising dopants is separated from the carrier substrate by an epitaxial layer and by an electrically insulating layer which ensures bonding between the donor substrate and the carrier substrate not comprising any such dopants.

[0054] Figure 2 The formation of the donor substrate 1 is illustrated.

[0055] The donor substrate 1 includes a seed substrate 100 made of a single-crystal semiconductor material such as silicon.

[0056] The seed crystal substrate 100 has 10 E15 at / cm 3 Such substrates are standard in particular in the microelectronics industry and are available at a cost-effective price.

[0057] A single crystal semiconductor epitaxial layer 101 is epitaxially grown on the seed crystal substrate 100. The epitaxial growth conditions are selected to avoid or at least minimize the presence of dopants in the layer 101. In any case, the concentration of the dopant in the layer 101 is lower than the concentration of the dopant in the seed crystal substrate 100. Preferably, the concentration of the dopant in the layer 101 is lower than 1 E 14 at / cm 3 , and if possible, have 1 E 13 at / cm 3 of magnitude.

[0058] In order to ensure good crystalline quality of the layer 101 , the material of said layer advantageously has a lattice parameter close to the lattice parameter of the seed substrate 100 , which serves as seed for the growth of the single-crystalline layer 101 .

[0059] In some embodiments, the epitaxial layer is formed of the same material as the seed substrate (without dopants).

[0060] The thickness of the undoped epitaxial semiconductor layer is between 10nm and 1000nm, which is greater than that of the Smart Cut TM The thickness of the layer transferred by the process.

[0061] The formation of this composite donor substrate makes it possible to limit the presence of dopants in the layer to be transferred from the donor substrate to the carrier substrate at a cost lower than that of an undoped bulk substrate. In particular, it is possible to use a seed substrate of lower quality than that of the conventionally used donor substrates, as long as the crystal quality of the layer to be transferred is epitaxially defined.

[0062] refer to Figure 3 , an electrically insulating layer 10 is formed on the undoped epitaxial semiconductor layer 101. This layer 10 makes it possible, in particular, to minimize the direct paths of atomic species during implantation (a phenomenon known as "channeling"). Furthermore, this layer 10 performs the function of a bonding layer between the donor substrate and the carrier substrate.

[0063] Particularly advantageously, the layer 10 is an oxide layer in order to ensure a good quality bond with the semiconductor material of the carrier substrate.

[0064] The layer 10 may in particular be formed by thermal oxidation of the undoped epitaxial layer 101. The layer 10 is therefore substantially free of dopants.

[0065] As an alternative, Figure 4As illustrated, the undoped epitaxial layer 101 is not formed directly on the seed crystal substrate 100 , but is formed on an intermediate layer 102 that is previously formed on the seed crystal substrate 100 .

[0066] Intermediate layer 102 is a single-crystalline semiconductor layer formed of a material different from that of the epitaxial layer. The material is advantageously selected to allow selective etching of undoped epitaxial layer 101 relative to intermediate layer 102, while having a lattice parameter sufficiently close to that of layer 101 to allow growth of layer 101 with good crystalline quality.

[0067] For example, if the material of the undoped epitaxial layer 101 is silicon, the material of the intermediate layer 102 is advantageously silicon germanium with a germanium content less than or equal to 30%.

[0068] The intermediate layer 102 may be epitaxially formed on the seed substrate 100. Preferably, the material of the intermediate layer also has a lattice parameter sufficiently close to that of the seed substrate 100 to allow growth of the intermediate layer 102 with good crystalline quality.

[0069] The thickness of the intermediate layer 102 may be between 10 nm and 100 nm.

[0070] Reference above Figure 3 The illustrated electrically insulating layer 10 is formed on an undoped epitaxial semiconductor layer 101 .

[0071] The following figures show an embodiment of a donor substrate comprising an intermediate layer 102, but it goes without saying that the description also applies to a donor substrate comprising Figure 3 The illustrated embodiment forms an epitaxial layer directly on a seed substrate.

[0072] refer to Figure 5 , ionic species are implanted (schematically shown by arrows) into the donor substrate through the electrically insulating layer 10 .

[0073] The implanted species typically include hydrogen and / or helium.

[0074] The dose and energy of the implanted seeds are chosen to form a weakened zone 11 in the undoped epitaxial layer 101. The weakened zone 11 defines a thin layer 12 to be transferred in the layer 101. The thickness of the layer 12 to be transferred may be between 4 nm and 100 nm.

[0075] refer to Figure 6 , a donor substrate 1 is bonded to a carrier substrate 2 via an electrically insulating layer 10 .

[0076] The carrier substrate 2 is a semiconductor substrate, for example made of silicon, having a high resistivity, for example greater than 500 Ω·cm, preferably greater than or equal to 1000 Ω·cm.

[0077] The carrier substrate is particularly advantageously a silicon substrate having a high interstitial oxygen content, i.e., a content greater than 20 ppma (for the definition of 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 denoted by the abbreviation "HiOi." Interstitial oxygen atoms readily precipitate during heat treatment, forming a large number of defects caused by oxygen precipitation, known as "bulk microdefects" (BMDs). These BMDs block dislocations generated during high-temperature heat treatment, which helps maintain the crystalline quality of the carrier substrate.

[0078] 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 precipitate interstitial oxygen and form the BMDs. This thermal treatment can typically be performed in a thermal cycle reaching a temperature of the order of 1000°C for 12 hours.

[0079] Furthermore, HiOi substrates typically include a large number of crystal defects known as COPs (an acronym for the term “crystal-derived 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, so as to allow oxygen to diffuse out of the substrate. In this case, this precipitation / diffusion thermal treatment should be performed before forming the electrically insulating layer on the carrier substrate.

[0080] Alternatively, one skilled in the art may choose a silicon substrate with a low or medium interstitial oxygen content (i.e., a content of less than 10 ppma, or a content between 10 and 20 ppma, respectively) as the carrier substrate. Such substrates are typically designated by the abbreviations "LowOi" and "MidOi," respectively. In this case, the above-described precipitation and / or diffusion heat treatment is not necessary.

[0081] The bonding can be supplemented by methods such as preparing the electrically insulating surface using oxygen plasma.

[0082] refer to Figure 7 , the donor substrate 1 is separated along the weakened zone 11. In a manner known per se, the separation can be caused by applying a mechanical stress in the vicinity of the weakened zone, by a heat treatment or by any other suitable means.

[0083] 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 10 and the transferred layer 12 is obtained.

[0084] The structure is then subjected to the finishing treatments conventionally applied to FDSOI substrates. These finishing treatments include, in particular, the thermal smoothing ("batch annealing") of the transferred layer as mentioned in the introduction.

[0085] In some embodiments, the smoothing process includes placing a batch of FDSOI structures in a furnace, slowly raising 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.

[0086] Although the thermal budget of the smoothing process is high enough to allow diffusion of the dopants present in the structure, the dopants in the seed structure have been kept sufficiently far away from the bonding interface by the epitaxial layer 101 and the electrically insulating layer 10 (which does not contain any such dopants) so as not to diffuse into the carrier substrate 2. Consequently, the resistivity of the carrier substrate is not affected, even in its portion close to the bonding interface.

[0087] Therefore, the FDSOI structure so formed is fully functional for RF applications (especially in the mmWave band).

[0088] Furthermore, at the end of the separation, the remaining part 1 ' of the donor substrate after separation can be recycled so as to allow the formation of a new donor substrate that can be used for a new transfer layer. Figure 7 As illustrated, the remaining portion 1 ′ of the donor substrate comprises the seed substrate 100 , the intermediate layer 102 (if present), and the portion 120 of the epitaxial layer 101 that has not yet been transferred to the carrier substrate.

[0089] refer to Figure 8 , a first step of recovery consists in etching the portion 120 not transferred from the epitaxial layer 101 selectively with respect to the intermediate layer 102. To this end, a wet etching can be performed by means of a suitable etching solution.

[0090] refer to Figure 9 The second step of recovery consists in etching the intermediate layer 102 selectively with respect to the seed substrate 100. To this end, wet etching can be achieved by means of a suitable etching solution.

[0091] Next, a new intermediate layer 102 ′ (see Figure 10 ) and the new undoped epitaxial layer 101 '(see Figure 11 ) to form a new donor substrate.

[0092] This recycling method is advantageous compared to recycling a donor substrate comprising an epitaxial layer directly on a seed substrate.

[0093] Specifically, in the case of a donor substrate in which an epitaxial layer is formed directly on a seed substrate, it is not possible to selectively etch the portion not transferred from the epitaxial layer relative to the seed substrate, because the material of the seed substrate and the epitaxial layer have similar compositions, differing only in their doping levels. In this case, recycling the remaining portion of the donor substrate requires polishing both sides of the remaining portion of the donor substrate using a chemical mechanical polishing (CMP) process to remove all portions not transferred from the epitaxial layer before growing a new epitaxial layer, followed by cleaning the surface of the seed substrate. Consequently, each recycling sequence consumes a portion of the thickness of the seed substrate, thereby limiting the number of possible uses of the substrate.

[0094] In contrast, when the donor substrate includes an intermediate layer between the seed substrate and the epitaxial layer that performs the function of an etch stop, the recycling method can be based solely on etching steps that do not consume material from the seed substrate. Therefore, the seed substrate can be reused indefinitely, reducing the cost of obtaining the donor substrate.

Claims

1. A method for manufacturing a semiconductor-on-insulator substrate for radio frequency applications, the method comprising the following steps: - forming a donor substrate (1) by epitaxially growing an undoped semiconductor layer (101) on a p-doped semiconductor seed crystal substrate (100); - forming an electrical insulating layer (10) on the undoped epitaxial semiconductor layer (101); - implanting ionic species through the electrically insulating layer (10) to form a weakened zone (11) in the undoped epitaxial semiconductor layer (101) defining a semiconductor thin layer (12) to be transferred; - providing a semiconductor carrier substrate (2) having a resistivity greater than or equal to 500 Ω·cm; - bonding the donor substrate (1) to the carrier substrate (2) via the electrically insulating layer (10); - separating the donor substrate (1) along the weakened zone (11) to transfer the semiconductor thin layer (12) from the donor substrate (1) to the carrier substrate (2), wherein, in the manufactured semiconductor-on-insulator substrate, diffusion of dopants from the donor substrate (1) into the carrier substrate (2) is prevented by the semiconductor thin layer (12) and the electrically insulating layer (10), so that the resistivity of the carrier substrate (2) is not affected.

2. The method according to claim 1, wherein The thickness of the undoped epitaxial semiconductor layer (101) is between 10 nm and 1000 nm.

3. The method according to claim 1 or 2, wherein: The seed substrate (100) is boron-doped.

4. The method according to claim 1 or 2, wherein: The step of forming the electrical insulating layer (10) comprises thermally oxidizing the material of the undoped epitaxial semiconductor layer (101).

5. The method according to claim 1 or 2, wherein: The step of forming a donor substrate (1) includes forming an intermediate layer (102) made of a material different from that of the epitaxial semiconductor layer (101) between the seed substrate (100) and the undoped epitaxial semiconductor layer (101), the material of the intermediate layer (102) being selected to allow the undoped epitaxial layer (101) to be selectively etched relative to the intermediate layer (102).

6. The method according to claim 5, wherein: The material of the undoped epitaxial layer (101) is silicon, and the material of the intermediate layer (102) is silicon germanium with a germanium content less than or equal to 30%.

7. The method according to claim 5, comprising the steps of: After the separation, the remaining portion of the undoped epitaxial semiconductor layer (101) is selectively etched relative to the intermediate layer (102), and then the intermediate layer (102) is selectively etched relative to the seed substrate (100), and a new donor substrate (1) is formed by sequentially forming a new intermediate layer (102') and a new undoped epitaxial layer (101') on the seed substrate (100).

8. The method according to claim 1 or 2, wherein: The thickness of the electrical insulating layer (10) is between 10 nm and 150 nm.

9. The method according to claim 1 or 2, wherein: The thickness of the transferred semiconductor layer (12) is between 4 nm and 300 nm.

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