Process for oxidizing a silicon layer

Abstract

The invention relates to a method for oxidizing a silicon layer comprising the following steps: supplying a substrate comprising a silicon layer; at least one implantation of sulfur atoms in at least one area of ​​the silicon layer; wet oxidation of said implanted silicon layer.

Description

Title of the invention: Process for oxidizing a silicon layer technical field

[0001] The present invention relates generally to the field of microelectronics. More particularly, it relates to a process for oxidizing a silicon layer. STATE OF THE ART

[0002] Oxidation is a very important step in the fabrication of silicon integrated circuits. It is used for various applications, in particular for creating isolation zones between different components of an integrated structure.

[0003] It is therefore necessary to have a good grasp of the oxidation processes.

[0004] Various techniques are known for obtaining silicon oxide, such as thermal oxidation in the presence of oxygen, known as dry oxidation, or thermal oxidation in the presence of water vapor, known as wet oxidation.

[0005] Thermal oxidation involves exposing silicon to high temperatures (generally between 800°C and 1200°C) in the presence of oxygen (dry phase) or water vapor (wet phase). The oxygen reacts with the silicon to form silicon dioxide (SiO2) on the surface.

[0006] It may be advantageous to try to modulate the oxidation rate of silicon, either by slowing it down or, more conventionally, by increasing it, in order to obtain a given oxide thickness more quickly. Methods for modulating the thermal oxidation rate of silicon are known. For example, doping silicon with elements such as phosphorus or boron makes it possible to accelerate the oxidation rate and obtain an oxidation ratio between 0.5 and 2: the oxidation ratio is defined as the ratio between the oxide thickness obtained with doping and the oxide thickness obtained under the same operating conditions without doping. Summary of the invention

[0007] The present invention aims to accelerate the rate of thermal oxidation of silicon by proposing a process which makes it possible in particular to achieve oxidation ratios much greater than 2 while maintaining a good quality of the oxidized layer.

[0008] The invention then relates to a process for oxidizing a silicon layer comprising the following steps: • implantation of sulfur atoms in at least one area of ​​the silicon layer; • wet oxidation of said implanted silicon layer.

[0009] Wet oxidation is understood to mean an oxidation step carried out in the presence of water vapor at a temperature higher than the ambient temperature, preferably above 700°C. This wet oxidation can take place either in the presence exclusively of water vapor or under a mixed atmosphere including both water vapor and dioxygen or water vapor and gaseous hydrogen chloride.

[0010] In a particularly surprising way, the inventors realized that combining doping with sulfur atoms in silicon and using wet oxidation resulted in a significant acceleration of oxidation compared to known techniques, achieving oxidation ratios well above 2. To achieve this, the oxidation must take place under a steam atmosphere (i.e., a dry atmosphere does not give the same results) and the sulfur atoms must be implanted in the silicon. It is therefore essential to create a sulfur-implanted zone, also referred to hereafter as a chamber, directly within the silicon.

[0011] In addition to the characteristics mentioned in the preceding paragraphs, the oxidation process according to the invention may have one or more additional characteristics from among the following, considered individually or in all technically possible combinations: • the sulfur atom implantation step is carried out so as to obtain a sulfur concentration in the implanted area strictly less than 5.1021 at / cm3 and preferably strictly less than 3.1021 at / cm3. • the substrate comprising a silicon layer further comprises an oxide layer on the silicon layer, the implantation then being carried out in such a way that the sulfur atoms are implanted under the oxide layer. • Wet oxidation is carried out: • in the presence of water vapor or • under an atmosphere including both water vapor and dioxygen or • under an atmosphere including water vapor and hydrogen chloride gas. • Wet oxidation is carried out at a temperature strictly above 700°C. • the implantation of sulfur atoms is carried out over a thickness greater than or equal to 10m. • The process comprises a plurality of implantation steps carried out successively and presenting implantation doses and / or tensions of different accelerations so as to obtain a uniform sulfur concentration over a given thickness. • The process according to a first embodiment comprises: • a masking step covering at least one area of ​​the silicon layer, • Sulfur atoms are implanted in at least one unmasked area of ​​the silicon layer, • a mask removal step preceding the wet oxidation step of said silicon layer.

[0012] The method according to a second embodiment in which the substrate comprising a silicon layer is a silicon-on-insulator (SOI) substrate, said method being implemented for the creation of at least one local insulation zone such as an insulation trench for example, comprising the following steps: • a masking step covering at least one area of ​​the silicon layer, • Sulfur atoms are implanted in at least one area of ​​the unmasked silicon layer intended to form the local insulation zone, so that the sulfur atoms are implanted under the oxide layer, • wet oxidation of said implanted silicon layer, • Removal of the oxide with a stop on the silicon layer.

[0013] The invention also relates to a device comprising an SOI substrate oxidized by the process according to the invention having at least one local isolation zone comprising sulfur.

[0014] According to one embodiment, the device according to the invention comprises a transistor formed on the upper silicon layer of the SOL substrate BRIEF DESCRIPTION OF THE FIGURES

[0015] Other features and advantages of the invention will become clear from the description given below, by way of example and not limitation, with reference to the accompanying figures, among which:

[0016] Figure 1 represents, in the form of a flowchart, the different steps of the process according to the invention.

[0017] Figures [Fig.2], [Fig.3] and [Fig.4] represent the different stages of the process of [Fig.1],

[0018] Figure 5 represents a concentration profile of sulfur atoms implanted during the implementation of the process according to the invention.

[0019] Figure 6 shows the oxide thickness obtained for samples made with different thicknesses of the layer implanted with sulfur after oxidation at different temperatures,

[0020] Figure 7 represents, in the form of a flowchart, the different steps of the process according to the invention.

[0021] Figures [Fig. 8], [Fig. 9] and [Fig. 10] represent the different stages of the process of [Fig. 7],

[0022] Figure 11 shows the oxide thickness obtained for samples made with different thicknesses of the layer implanted with sulfur and an initial oxide thickness after oxidation at different temperatures,

[0023] Figure 12 shows the oxide thickness obtained for samples made with the same thickness of the layer implanted with sulfur at different sulfur concentrations after oxidation at 850°C for different durations,

[0024] Figure 13 shows the evolution of the thickness of oxide formed as a function of time for different samples,

[0025] Fig. 14 shows the evolution of the oxidation rate for different samples,

[0026] Fig. 15 and Fig. 16 illustrate examples of application of the oxidation process according to the invention.

[0027] For clarity, identical or similar elements are identified by identical reference signs throughout the figures. DETAILED DESCRIPTION OF THE INVENTION

[0028] Fig. 1 represents a flowchart illustrating the different steps of the process 100 of oxidation of a silicon layer according to a first embodiment of the invention.

[0029] As shown in [Fig. 2], the process 100 begins with a step 101 of supplying a substrate 200 comprising a silicon layer. Here, the substrate 200 is a bulk substrate made entirely of monocrystalline Si, such that the Si layer and the substrate are indistinguishable. However, as will be seen later, the invention is not limited to the Si bulk substrate, and the silicon layer can, for example, be a layer of monocrystalline silicon belonging to a silicon-on-insulator (SOI) substrate or a silicon layer deposited on any stack of layers.

[0030] The process according to the invention continues with a step 102 ([Fig. 3]) corresponding to the formation of a sulfur atom cell 201 in the Si layer 200. In other words, at least one zone 201 of the Si layer is doped with sulfur atoms over a given thickness e measured perpendicular to the plane of the Si layer. The formation of the sulfur cell 201 in the silicon layer is obtained by at least one ionic implantation step of sulfur atoms in the Si 200 layer. Depending on the desired thickness e, it may be useful to perform several successive implantations of sulfur atoms in the Si 200 layer to obtain a homogeneous sulfur concentration throughout the thickness e (also referred to as the depth). For illustration, [Fig. 5] shows the 300 profile of sulfur atom concentration (in at / cm³) implanted to obtain a homogeneous concentration of 1021 at / cm³ over a depth of 150 nm. Such a 300 profile is obtained by means of three successive implantations of sulfur ions in the Si layer: • A first implantation 301 with an accelerating voltage of 80kV, a dose of 1.5 x 10¹⁶ cm², a tilt angle of 0° and a twist angle of 0°, • A second implantation 302 with an accelerating voltage of 24kV, a dose of 5.1015 cm2, a tilt angle of 7° and a twist angle of 27°, • A third implantation 303 with an acceleration voltage of 2.25kV, a dose of 2.5.1015 cm2, a tilt angle of 7° and a twist angle of 27°.

[0031] For the record, the two implantation angles necessary to parameterize the ion beam with respect to the crystal lattice can be defined. These angles are the tilt T and the twist R. The tilt T is the angle between the ion beam and the normal to the surface of the target substrate. The twist R is the angle between the incident beam and the axis of the substrate's flat or notch.

[0032] The process 100 continues with a step 103 ([Fig. 4]) of oxidation of the silicon layer 200, which is overlain by the sulfur box 201, corresponding to a sulfur-doped silicon layer. According to the invention, the oxidation is a moist thermal oxidation (at a temperature greater than or equal to 700°C) (i.e., in the presence of a water vapor atmosphere). The silicon layer 200 and the box 201 react with the oxidizing agent to form a SiO2 layer 203 by consuming the silicon. The Si / SiO2 interface will then be located below the initial surface 204 of the sulfur box 203 and will penetrate the initial, undoped silicon layer. As is known, the fraction of the oxide layer thickness located below the initial surface 204 represents about 46% of the total oxide thickness and the fraction of the oxide thickness located above the initial surface 204 represents about 54% of the total oxide thickness.

[0033] According to a first experiment and in order to show the advantages of the process according to the invention, samples Q2, Q3 and Q4 were made with different thicknesses e of the layer 201 implanted with sulfur.

[0034] Sample Q1 will be considered in what follows as a reference sample without sulfur implantation. Sample Q2 has an implantation thickness of 1 nm. Sample Q3 has an implantation thickness of 50 nm and sample Q4 has a implantation thickness of 150nm. The sulfur concentration in samples Q2, Q3 and Q4 is equal to 1021 at / cm3.

[0035] Each of the samples Q1, Q2, Q3 and Q4 is then subjected to wet oxidation for Ih at three different temperatures, respectively, 850°C, 950°C and 1050°C.

[0036] Figure 6 shows the oxide thickness obtained for each of the samples after oxidation at each of the aforementioned temperatures. For each representation, the reference to Onm corresponds to the initial surface area of ​​the sulfur chamber for samples Q2, Q3, and Q4, and to the initial surface area of ​​the silicon layer for the reference sample Q1 (i.e., without implantation). The oxidized zone is in each case decomposed into two parts: the oxide thickness formed in the sulfur chamber and the oxide thickness formed above the upper initial surface of the sulfur chamber (or above the upper initial surface of the Si layer in the case of sample Q1) and below the lower initial surface of the sulfur chamber (or below the lower initial surface of the Si layer in the case of sample Q1).Under each thickness of samples Q2, Q3 and Q4 is represented a multiplication factor corresponding to the ratio between the oxide thickness of the given sample and the oxide thickness of the reference sample QL.

[0037] Regardless of the oxidation temperature, the ratio increases as the thickness of the sulfur chamber increases, this ratio being systematically greater than 1. Thus, the presence of sulfur allows, for the same oxidation time, a greater oxide thickness than in the case of the reference sample. Furthermore, oxide growth accelerates with increasing chamber thickness: for example, at 850°C, the ratio is 1.7 for a 1 nm chamber and increases to 5.2 for a 150 nm chamber. The process according to the invention therefore makes it possible to effectively modulate the thermal oxidation rate of silicon and to achieve oxidation ratios well above 2. As is known, the oxidation rate increases with the temperature at which the oxidation is carried out. It should also be noted that the use of dry oxidation does not yield the same results.

[0038] Fig. 7 represents a flowchart illustrating the different steps of the process 400 of oxidation of a silicon layer according to a second embodiment of the invention.

[0039] As shown in [Fig.8], the process 400 begins with a step 401 of supplying a substrate 500 comprising a silicon layer 501 (here in the form of a bulk substrate entirely in single-crystal Si) whose upper surface is oxidized (presence of an oxide layer 502) over a thickness el, here equal to 20nm.

[0040] The process 400 according to the invention continues with a step 402 ([Fig. 9]) corresponding to the creation of a compartment 503 of sulfur atoms in the stack formed by the Si layer 501 and the oxide layer 502. It should be noted that in [Fig. 9], the sulfur-implanted zone covers both the thickness el of the oxide layer and part of the thickness of the Si layer (we will see the importance of this characteristic later). In other words, at least one zone 504 of the unoxidized Si layer is doped with sulfur atoms. The total thickness e2 of the sulfur box 503 is measured perpendicular to the plane of the Si layer. As before, the realization of the sulfur box 503 is obtained by at least one ionic implantation step of sulfur atoms in the Si layer 501 and the oxide layer 502 above it.As mentioned for the first embodiment, depending on the desired thickness e2, it may be useful to carry out several successive implantations of sulfur atoms in the Si 501 layer in order to obtain a homogeneous concentration of sulfur over the entire thickness e2.

[0041] The process 400 continues with a step 403 ([Fig. 10]) of oxidation of the silicon layer 501 overlain by the sulfur chamber 504, corresponding to a sulfur-doped silicon layer and the sulfur-implanted oxide layer. According to the invention, the oxidation is a moist thermal oxidation (at a temperature greater than or equal to 800°C) (i.e., in the presence of a water vapor atmosphere). The silicon layer 501 and the chamber 504 react with the oxidizing agent to form a SiO2 layer 505 by consuming the silicon.

[0042] According to a second experiment and in order to illustrate the properties of this second embodiment, samples Q2, Q3 and Q4 were made with different thicknesses e2 of the 503 layer implanted with sulfur (always with an initial oxide layer of thickness el equal to 20nm).

[0043] Sample Q1 will be considered in the following as a reference sample without sulfur implantation but with an initial oxide layer 20 nm thick. Sample Q2 has an implantation thickness of 100 nm. Sample Q3 has an implantation thickness of 50 nm and sample Q4 has an implantation thickness of 150 nm. The sulfur concentration in samples Q2, Q3 and Q4 is 1021 at / cm³.

[0044] Each of the samples Q1, Q2, Q3 and Q4 is then subjected to wet oxidation for Ih at three different temperatures, respectively, 850°C, 950°C and 1050°C.

[0045] Figure 11 shows the oxide thickness obtained for each of the samples after oxidation at each of the aforementioned temperatures. For each representation, the reference to Onm corresponds to the initial surface area of ​​the initial oxide layer for samples Q1, Q2, Q3, and Q4. The objective of these experiments is to see if The acceleration of oxidation due to sulfur occurs only in silicon or if the same phenomenon is observed in the initial oxide layer implanted in sulfur.

[0046] The oxidized zone is in each case decomposed into three parts: the thickness of oxide formed in the sulfur chamber, the initial oxide thickness, and the thickness of oxide formed above the upper surface of the initial oxide layer and below the lower surface of the sulfur chamber (or below the lower surface of the initial oxide layer in the case of sample Q1). Under each thickness of samples Q2, Q3, and Q4, the multiplication factor corresponding to the ratio between the oxide thickness of the given sample and the oxide thickness of the reference sample Q1 is shown.

[0047] Regardless of the oxidation temperature, the ratio is observed to be equal to 1 when the sulfur cavity is formed exclusively within the initial oxide layer (i.e., the sulfur cavity thickness is 1 nm, therefore less than the initial oxide thickness of 20 nm). In other words, when sulfur implantation occurs only in the initial oxide layer, no effect on oxidation acceleration is observed. It is therefore the presence of sulfur in the unoxidized silicon layer that causes oxidation acceleration. This observation is confirmed by the other ratios, which are strictly greater than 1 when the sulfur cavity thickness increases such that sulfur is present within the silicon. For example, at 850°C, the ratio is 2.3 for a 50 nm cavity (50 nm being significantly greater than the initial oxide thickness of 20 nm) and increases to 4.9 for a 150 nm cavity (150 nm also being greater than the initial oxide thickness of 20 nm).The process according to the invention thus makes it possible to effectively modulate the thermal oxidation rate of silicon and to achieve oxidation ratios well above 2 when sulfur is implanted in the silicon (the chamber must therefore have a thickness strictly greater than the initial oxide thickness). As previously observed and as is known, it is also observed that the oxidation rate increases with the temperature at which the oxidation is carried out.

[0048] The inventors also analyzed the effect of the sulfur concentration in the implanted chamber on the process according to the invention. Thus, according to a third experiment and with reference to [Fig. 3], samples Q2, Q3, and Q4 were prepared with the same implanted layer thickness of 150 nm and different sulfur concentrations in this layer. Sample Q1 will be considered in what follows as a reference sample. Sample Q2 has a sulfur concentration of 5 x 10²⁰ at / cm³. Sample Q3 has a sulfur concentration of 2 x 10²⁰ at / cm³. Sample Q4 has a sulfur concentration of 5 x 10²⁰ at / cm³.

[0049] Each of the samples Q1, Q2, Q3 and Q4 is then subjected to wet oxidation at 850°C for 1h, 30min and 15min respectively.

[0050] Figure 12 shows the oxide thickness obtained for each of the samples after oxidation for each of the aforementioned oxidation times. For each of the representations, the reference to Onm corresponds to the initial surface area of ​​the sulfur chamber for samples Q2, Q3 and Q4 and to the initial surface area of ​​the silicon layer for the reference sample Q1 (i.e. without implantation).

[0051] As can be seen in [Fig. 12], beyond a certain concentration, oxidation becomes less efficient and delamination occurs. Preferably, the sulfur concentration in the implanted area is strictly less than 5 x 10²¹ at / cm³ (the sulfur concentration at which delamination is observed in [Fig. 12]) and preferentially strictly less than 3 x 10²¹ at / cm³. For all other concentrations, an oxidation ratio strictly greater than 1 is observed, and therefore an acceleration of oxidation is linked to the presence of sulfur.

[0052] Electrical characterizations of type C(V) seem to show that the oxides obtained via sulfur implantation according to the invention possess electrical properties similar to those of the oxides obtained without sulfur: in other words, the oxides obtained via sulfur implantation according to the invention appear to show no electrical degradation compared to the oxides obtained without sulfur.

[0053] Figure 13 shows the evolution of the oxide thickness formed as a function of time for samples Q1, Q2, and Q3, as well as for an additional sample with a sulfur concentration of 1 x 10²¹ at / cm³. Figure 14 shows the evolution of the oxidation rate for samples Q1, Q2, and Q3 from Figure 12. An acceleration of oxidation is observed during the first few minutes of oxide formation, and the oxidation rate reaches a saturation point beyond a certain depth. In other words, the benefit of sulfur doping in increasing the oxidation rate is particularly noticeable during the first few minutes of oxidation.

[0054] Figures 15 and 16 illustrate examples of application of the oxidation process according to the invention.

[0055] Figure 15 shows the use of the process according to the invention within the framework of LOCOS or "Local Oxidation of Silicon" technology. To this end, a silicon 600 substrate is used. In accordance with the invention, sulfur 602 cells are then implanted (two sulfur 602 cells are shown here). The implantation is preceded by a masking step defining masked areas 601.

[0056] Masking operations are obtained by LOCOS lithography for the production of masks (for example in resin) to protect certain regions of the substrate 600.

[0057] The implantation step is followed by a resin removal step, for example by stripping or pickling according to English terminology.

[0058] According to the invention, the silicon areas implanted with sulfur 602 and the unimplanted silicon areas 601 are then subjected to wet oxidation so as to obtain a layer 603 of oxidized silicon with thicker oxidation zones 604 at the locations of the sulfur 602 cells. The thicker oxidation zones 604 will in particular provide insulation between components and are obtained by a reduced number of steps.

[0059] Fig. 16 shows the use of the process according to the invention within the framework of an STI (Shallow Trench Isolation) technology.

[0060] To do this, we start from a silicon substrate 600 of the silicon on insulator SOI (“Silicon On Insulator”) type 700 including a layer of monocrystalline silicon 703 above a buried insulating layer 702 commonly designated by those skilled in the art under the Anglo-Saxon name of “BOX” above a lower region 701 of silicon.

[0061] In accordance with the invention, sulfur cells 705 are then implanted (two sulfur cells 705 are shown here). As mentioned previously, for the process according to the invention to be effective, it is desirable that the sulfur implantation not be carried out solely in the oxide 702; this is why the sulfur implantation is carried out here in the upper Si layer 703, in the buried oxide layer 702, and also in the Si layer 701. The implantation is preceded by a masking step defining masked areas 704.

[0062] The masking operations are obtained by STI lithography for the production of resin masks to define the non-implanted patterns (masked areas 704).

[0063] The implantation step is followed by a resin removal step, for example by pickling or stripping according to English terminology.

[0064] According to the invention, the silicon and oxide zones implanted with sulfur 705 and the unimplanted silicon and oxide zones 704 are then subjected to wet oxidation so as to obtain a layer 706 of oxidized silicon with thicker oxidation zones 707 (i.e. with greater growth upwards and downwards) at the locations of the sulfur boxes 705.

[0065] The thick oxidation zones 707 are then subjected to a chemical mechanical polishing (CMP) process, stopping on the upper surface of the upper Si layer 703 for Remove excess oxidized surface material and obtain planarized insulating areas 708. Wet cleaning can also be performed to remove material residues and prepare the surface for further manufacturing processes. Areas 707 can, in particular, serve as isolation trenches, allowing, for example, the isolation of a transistor fabricated on the top layer of Si 703 from other transistors.

[0066] It should be noted that equivalent durations are represented in each of Figures 6, Figures 11 and 12 illustrate the oxidation time required for a reference sample to achieve the same oxidation thickness as the sulfur-implanted sample under the same oxidation conditions.

Claims

Demands

1. A process for oxidizing a silicon layer comprising the following steps: • supplying (101) a substrate (200) comprising a silicon layer; • at least one implantation (102) of sulfur atoms in at least one area (201) of the silicon layer; • wet oxidation (103) of said implanted silicon layer.

2. The method according to claim 1 characterized in that the sulfur atom implantation step is carried out so as to obtain a sulfur concentration in the implanted area strictly less than 5.1021 at / cm3 and preferably strictly less than 3.1021 at / cm3.

3. A process according to any one of the preceding claims characterized in that the wet oxidation is carried out: • in the presence of water vapor or • under an atmosphere including both water vapor and dioxygen or • under an atmosphere including water vapor and gaseous hydrogen chloride.

4. A method according to any one of the preceding claims wherein the substrate comprising a silicon layer further comprises an oxide layer on the silicon layer, the implantation then being carried out so that the sulfur atoms are implanted under the oxide layer.

5. A process according to any one of the preceding claims characterized in that the wet oxidation is carried out at a temperature strictly above 700°C.

6. A method according to any one of the preceding claims characterized in that the implantation of sulfur atoms is carried out over a thickness greater than or equal to 10m.

7. A method according to any one of the preceding claims, characterized in that it comprises a plurality of implantation steps carried out successively and having implantation doses and / or different acceleration voltages so as to obtain a uniform sulfur concentration over a given thickness.

8. A method according to any one of the preceding claims characterized in that it comprises: - a step of masking at least one area of ​​the silicon layer, - the implantation of sulfur atoms is carried out in at least one unmasked area of ​​the silicon layer, - a step of removing the mask preceding the wet oxidation step of said silicon layer.

9. A method according to any one of claims 1 to 7 wherein the substrate comprising a silicon layer is a silicon-on-insulator (SOI) substrate, said method being implemented for the realization of at least one local isolation zone, such as an isolation trench for example, comprising the following steps: - a masking step of at least one zone of the silicon layer, - the implantation of sulfur atoms is carried out in at least one zone of the unmasked silicon layer intended to form the local isolation zone, so that the sulfur atoms are implanted under the oxide layer, - wet oxidation of said implanted silicon layer, - removal of the oxide with a stop on the silicon layer.

10. Device comprising an SOI substrate oxidized by the process according to the preceding claim having at least one local isolation zone comprising sulfur.

11. Device according to the preceding claim characterized in that it comprises a transistor formed on the upper silicon layer of the SOI substrate.

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