Method for etching a substrate comprising a thin surface layer to improve the thickness uniformity of said layer

Abstract

The present invention relates to a method for etching a main surface of a thin layer of a substrate, the method comprising: immersing the substrate in an etching tank so as to expose the main surface to an etchant, the substrate being oriented relative to the tank in such a way as to compensate for thickness non-uniformities of the thin layer: when the substrate is introduced into the tank, the main surface is gradually immersed from an initial introduction point to a final introduction point according to an introduction speed, and when the substrate is withdrawn from the tank, the main surface is gradually revealed from an initial exit point to a final exit point according to an exit speed, the method being characterized in that: the introduction speed is selected in such a way that the main surface is etched according to a first non-uniform profile between the initial introduction point and the final introduction point, and / or the exit speed is selected in such a way that the main surface is etched according to a second non-uniform profile between the initial exit point and the final exit point.

Description

Technical Field

[0001] The present invention relates to the fields of microelectronics, optoelectronics, photonics, etc. In particular, the present invention relates to a method for etching a substrate using a chemical bath multi-wafer cleaner; the present invention is particularly applicable to SOI ("silicon on insulator") substrates including a very thin silicon surface layer. Background Art

[0002] An increasing number of applications based on SOI substrates require very high uniformity in the thickness of thin silicon surface layers (which are intended to house components or serve as their support). For example, in digital applications, thin layers of FDSOI ("fully depleted SOI") substrates must exhibit very low thickness variations, as these variations affect the threshold voltage of transistors generated on the thin layers. In photonics applications, the performance of filter or modulator devices is also severely affected by non-uniformity in the thickness of the thin layers of the SOI substrate.

[0003] As a result, the specifications in terms of thickness and uniformity have become very demanding: for layers typically less than 50 nm thick, the expected within-wafer (WiW) and wafer-to-wafer (WtW) non-uniformities are less than a few angstroms, typically less than 4 angstroms. This uniformity is difficult to achieve because the successive steps in producing SOI substrates result in cumulative contributions to the non-uniformity of thin layers.

[0004] One known solution for correcting non-uniformities in the thickness of thin layers is to locally etch the layer using, for example, a plasma etching method as described in document US20140234992 or a cluster ion beam etching method as described in document WO2013003745. However, this type of solution has the following disadvantages: etching the surface of the thin layer creates amorphous silicon surface areas that are prone to causing electrical problems and therefore must be removed. The removal of the amorphous areas leads to an increase in surface roughness, which adversely affects the performance of devices manufactured on the thin layer.

[0005] Document WO2004015759 proposes an alternative solution for localized sacrificial thermal oxidation, which locally consumes a greater or lesser thickness of a thin layer in order to correct its thickness non-uniformity. A disadvantage of this method is that local temperature gradients cannot be easily introduced into the silicon layer: therefore, the resolution of the non-uniformity correction may be limited.

[0006] Purpose of the Invention

[0007] The present invention relates to an alternative to the prior art solutions and aims to completely or partially overcome the aforementioned disadvantages. Specifically, the invention relates to a method for producing a thin layer with the aim of achieving improved thickness uniformity of the layer, the implementation of which is straightforward and reproducible. This production method can advantageously be implemented to produce SOI structures having very thin silicon surface layers. Summary of the Invention

[0008] The invention relates to a method for etching a main surface of a substrate comprising a thin surface layer, said main surface corresponding to the free face of said thin layer, said method comprising the steps of: immersing said substrate in an etching bath so as to expose said main surface to an etchant, said substrate being oriented relative to said bath in such a way as to compensate for the non-uniformity of the thickness of said thin layer:

[0009] - when the substrate is introduced into the tank, the main surface is gradually immersed from the initial introduction point to the final introduction point according to the introduction speed, and

[0010] - when the substrate is removed from the tank, the main surface gradually emerges from an initial removal point to a final removal point according to the removal speed,

[0011] The method is characterized in that:

[0012] - the introduction speed is selected in such a way that the main surface is etched according to a first non-uniform profile between the initial introduction point and the final introduction point, and / or

[0013] - the removal rate is selected in such a way that the main surface is etched according to a second non-uniform profile between the initial removal point and the final removal point.

[0014] Other advantageous and non-limiting features according to the invention, alone or in any technically feasible combination:

[0015] the first etch profile defines a thickness variation of greater than or equal to 0.15 nm or even greater than or equal to 0.2 nm etched on the major surface between the initial introduction point and the final introduction point, and / or

[0016] The second etch profile defines a thickness variation of greater than or equal to 0.15 nm or even greater than or equal to 0.2 nm etched on the major surface between the initial removal point and the final removal point;

[0017] The initial introduction point corresponds to the final removal point, and the final introduction point corresponds to the initial removal point;

[0018] The introduction speed and / or the removal speed is between 25 cm / s and 0.1 cm / s, preferably between 10 cm / s and 0.5 cm / s;

[0019] When the substrate is introduced into the tank and removed from the tank, the introduction speed and / or the removal speed, respectively, are non-constant;

[0020] The etching tank contains SC1 ("Standard Clean 1") solution, which has ammonia, hydrogen peroxide and deionized water in a ratio of 1 / 2 / 20 respectively;

[0021] The substrate is an SOI substrate, a surface thin layer of the SOI substrate having a thickness of less than 50 nm and arranged on an insulating layer, which is arranged on a carrier substrate made of silicon;

[0022] The thin layer is made of single crystal silicon. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Other features and advantages of the present invention will become apparent from the following detailed description of the invention given with reference to the accompanying drawings, in which:

[0024] - Figure 1 A substrate suitable for treatment with the production method according to the invention is shown;

[0025] - Figure 2 The steps of a method known from the prior art for transferring a thin layer to a carrier substrate are shown;

[0026] - Figure 3 a diagram showing the thickness non-uniformity of a thin layer of a substrate suitable for being treated with the production method according to the invention;

[0027] - Figure 4a and Figure 4b Simplified diagrams respectively show when a substrate is introduced into an etching tank and when the substrate is taken out of the etching tank in the etching method according to the present invention;

[0028] - Figure 5a A diagram showing a non-uniform etch profile on a thin layer of a substrate obtained according to the etching method of the present invention;

[0029] - Figure 5b shows the normalized etching gradients in the north / south profile along the lines numbered from 1 to 5 obtained according to the etching method of the invention on a thin layer of a substrate;

[0030] - Figure 6A box plot illustrating an etch thickness gradient in one example of a non-uniform etch profile obtained on multiple substrates processed with an etching method according to the present invention is shown; the box plot also illustrates an etch thickness gradient used with a conventional etch tank. DETAILED DESCRIPTION

[0031] Throughout the specification, like reference numerals may be used for like elements in the accompanying drawings. The drawings are schematic representations and, for the sake of clarity, are not drawn to scale. Specifically, the thickness of a layer along the z-axis is not proportional to the lateral dimensions along the x- and y-axes; the relative thicknesses of the layers relative to one another are not necessarily taken into account in the drawings.

[0032] The present invention relates to a method for etching a main surface 1a of a substrate 1. Specifically, the main surface 1a is the front side of the substrate 1, that is, the side whose physical properties (roughness, uniformity, cleanliness, etc.) are closely monitored with regard to component production.

[0033] The etching method is a wet etching method, which includes immersing the substrate 1 in a liquid etching tank 100 ( Figure 4a ) to expose the main surface 1a to the etchant.

[0034] The substrate 1 includes a carrier substrate 20 ( Figure 1 The invention is described in the specific case of a thin surface layer 11 on a substrate 11. In this case, the main surface 1a corresponds to the free face of the thin layer 11 on which the component is to be produced. The thin layer 11 is advantageously made of a single-crystal material with a very low defect density.

[0035] As is known per se, the substrate 1 can be formed by a thin layer 11 arranged directly on a carrier substrate 20 ( Figure 1 (a)), or it may include an intermediate layer 30 ( Figure 1 (b)). The intermediate layer 30 may be insulating, as in the case of an SOI type substrate 1. Alternatively, the intermediate layer 30 may be a semiconductor or conductive substrate, depending on the desired properties of the substrate 1.

[0036] There are many known methods for transferring a single-crystal thin layer 11 to a carrier substrate 20, including SmartCut TM Method based on the implantation of light species in order to create buried weakened planes 12 ( Figure 2 (a)). This method is also based on direct bonding ( Figure 2 (b)).

[0037] The donor substrate 10 and the carrier substrate 20 are preferably each in the form of a circular wafer having a diameter of 200 mm or 300 mm, or even 450 mm, and a thickness typically between 500 and 900 microns.

[0038] Of course, when an intermediate layer 30 is required in the final stack of substrates 1, either (or both) of the substrates 10, 20 may include all or part of the intermediate layer 30 on the face to be joined. Separation along the buried weakened plane 12 allows the thin layer 11 to be transferred from the donor substrate 10 to the carrier substrate 20 ( Figure 2 (c)). Immediately after separation, the free surface of thin layer 11 exhibits a high level of roughness (approximately 40 to 80 angstroms measured using an atomic force microscope on a 30×30 micron scan). Among the surface finishing techniques, sacrificial oxidation heat treatment, etching, and / or surface reconstruction can be performed, in particular, in a neutral or reducing atmosphere. These treatments are typically performed at high temperatures, depending on the material constituting thin layer 11. For example, for a silicon layer, oxidation can be performed at temperatures between approximately 750° C. and 1100° C., and smoothing can be performed at temperatures between 950° C. and 1250° C. in a neutral or reducing atmosphere.

[0039] After these finishing treatments, a substrate 1 is obtained having a thin surface layer 11. As mentioned in the introduction, the specifications with regard to the thickness uniformity of said layer 11 become increasingly demanding, especially as its thickness decreases.

[0040] The Applicant has recognized that the thickness non-uniformity of the thin layer 11 obtained using the Smart Cut method is due in particular to two contributions of non-concentric symmetry, especially in the case of layers 11 with very low thicknesses of less than 50 nm or even less than 20 nm.

[0041] A first contributing factor to the non-uniformity of the thin layer 11 results from natural variations in the successive steps performed to produce the substrate 1 and leads to unpredictable ("random") variations in the thickness of the layer 11 that are therefore difficult to eliminate.

[0042] The second contributing factor comes from production steps with non-concentric symmetry: for example, ion implantation, separation at buried weakening planes and / or batch heat treatment involving supports that locally hold substrate 1. This contributing factor produces a more or less pronounced inhomogeneity in the thickness of thin layer 11 between the base points N, S, E, O of substrate 1. For example, at Figure 3 In the north-south direction, a thickness non-uniformity of about 0.6 nm can be seen between the two base points N (north) and S (south). Note that Figure 3The values in are in angstroms. Layer 11 is thinner near point S and thicker near point N. This inhomogeneity profile is repeated along essentially all imaginary lines drawn parallel to the north-south direction. In other words, the northern region of layer 11 (the region near point N) appears thicker than the southern region (near point S).

[0043] Notice, Figure 3 The thickness non-uniformity diagram in is established based on measurements by ellipsometry or reflectometry of the thickness of the thin layer 11. In this example, the thin layer 11 is made of silicon and has an average thickness of 12 nm.

[0044] The above steps for producing non-uniformity with non-concentric symmetry are performed simultaneously in batches of dozens or even hundreds of substrates 1 in a multi-wafer tool. In order to ensure traceability and reproducibility of performance in each step across different batches of substrates 1, it is common practice to arrange all substrates 1 in the same position, which can be identified by the presence of a flat feature or notch feature 22 formed in the edge of each substrate 1. In practice, the notch feature will be located at the top (north cardinal point) or the bottom (south cardinal point - e.g. Figure 3 Thus, the non-uniformity profile appears in a relatively reproducible manner across all batches of substrates 1 processed simultaneously or continuously in various steps.

[0045] In particular, the etching method according to the invention addresses the mentioned second contributing factor to thickness non-uniformity.

[0046] As described above, the purpose of the etching method is to expose the main surface 1 a of the substrate 1 to an etchant by immersing it in the bath 100 containing a liquid solution.

[0047] The substrate 1 is oriented relative to the etching tank 100 so that when the substrate 1 is introduced into the tank 100, the main surface 1a is gradually immersed from the initial introduction point PII to the final introduction point PFI, which reflects the total immersion of the substrate 1 ( Figure 4a ). In other words, the main surface 1a forms a non-zero angle with the surface of the solution contained in the tank 100. Typically, this angle is about 90°.

[0048] As S is removed from the groove 100, the main surface 1a gradually emerges from the initial removal point PIS to the final removal point PFS, which reflects the total removal of the substrate 1 ( Figure 4b ).

[0049] According to the etching method of the invention, the speed of introducing the substrate 1 into the trench 100 and / or the speed of removing the substrate 1 from the trench 100 is selected so as to produce a non-uniform etching profile on the main surface 1 a .

[0050] More specifically, the introduction rate can be selected so as to etch the main surface 1a according to a first non-uniform profile between the initial introduction point PII and the final introduction point PFI. Alternatively or additionally, the removal rate can be selected so as to etch the main surface 1a according to a second non-uniform profile between the initial removal point PIS and the final removal point PFS.

[0051] Advantageously, the first etch profile defines a thickness variation (or gradient) greater than or equal to 0.15 nm or even greater than or equal to 0.2 nm etched on the main surface 1 a between the initial point of introduction PII and the final point of introduction PFI.

[0052] Similarly, the second etch profile preferably defines a thickness variation (or gradient) of greater than or equal to 0.15 nm or even greater than or equal to 0.2 nm etched on the main surface 1 a between the initial removal point PIS and the final removal point PFS.

[0053] The initial introduction point PII may correspond to the final removal point PFS, and the final introduction point PFI may correspond to the initial removal point PIS.

[0054] Alternatively, in order to more finely adjust the non-uniform etching profile to a specific type of thin layer 11 thickness non-uniformity, a rotation of a defined angle may be applied to the substrate 1 so that the introduction points PII, PFI and the removal points PIS, PFS differ from each other.

[0055] According to yet another option, the etching method may be applied multiple times in succession while rotating the substrate 1 between each iteration, in order to fine-tune the non-uniform etching profile in each iteration to a specific type of thin layer 11 thickness non-uniformity.

[0056] The speed at which substrates 1 are introduced into and removed from a liquid-chemical tank multi-wafer cleaning or etching apparatus is typically high. For example, for a 300 mm diameter substrate, the speed is approximately 30 cm / s. Thus, each substrate 1 enters the tank in less than 2 seconds, achieving the best possible processing uniformity during this step.

[0057] In contrast to this principle, in the etching method of the present invention, at least one of the speeds between the speed of introducing the substrate 1 (or substrates 1) and the speed of removing the substrate 1 (or substrates 1) is advantageously reduced: it can be between 25 cm / s and 0.1 cm / s, or even more specifically between 10 cm / s and 0.5 cm / s. As will be explained in more detail below in a preferred embodiment, reducing the speed at which the substrate 1 is introduced into the tank 100 and / or removed from the tank 100 makes it possible to produce a non-uniform etching profile and compensate for the thickness non-uniformity of the thin layer 11 according to the aforementioned second contributing factor.

[0058] The introduction speed and / or the removal speed may also be chosen to be non-constant in order to compensate for a non-linear inhomogeneity profile over the diameter of the substrate 1 .

[0059] According to a preferred embodiment, the thin layer 11 is made of single crystal silicon, and the solution in the etching tank 100 is an SC1 ("Standard Clean 1") solution based on deionized water, ammonia (NH3) and hydrogen peroxide (H2O2). The SC1 solution, which is commonly used to clean silicon wafers, is particularly effective in removing particulate contaminants from the treated surface.

[0060] The etching method according to the present invention uses, for example, an SC1 solution, wherein the etching rate of the silicon thin layer 11 is approximately 0.5 nm / min. The volume ratios of the three components of the solution (NH3 / H2O2 / H2O) are 1 / 2 / 20, and the bath temperature is 70°C. Megasonic stirring of the solution is also advantageous.

[0061] By way of example, Figure 5a The non-uniform etch profile obtained on a thin layer 11 of a substrate 1 having a diameter of 300 mm in an etching tank SC1 as described above is shown, wherein the substrate 1 is introduced into the tank at a speed of 23 cm / s, the residence time in the tank is 90 s, and the removal rate is 0.67 cm / s (i.e., there is 45 s between the occurrence of the initial removal point PIS and the occurrence of the final removal point PFS). In this example, the initial introduction point PII coincides with the final removal point PFS, and the final introduction point PFI coincides with the initial removal point PIS; the recess feature 22 is located at the final introduction point PFI (also the initial removal point PIS).

[0062] More specifically, the removal rate is reduced; thus, when removed from the tank, substantially Figure 5a The non-uniform etching profile (or the second non-uniform etching profile as described above) is shown. The etching thickness gradient between the two introduction points PII and PFI (or between the two removal points PIS and PFS) is about 3 angstroms: the etching thickness is larger at the initial introduction point PII (or the final removal point PFS) and smaller at the final introduction point PFI (or the initial removal point PIS).

[0063] Figure 5b The normalized etching gradient in the north / south profile along the lines numbered from 1 to 5 on a substrate 1 that has undergone the etching method according to the aforementioned example is illustrated. The etching thickness gradient between two introduction points PII, PFI (or between two removal points PIS, PFS) is found to be approximately 3 angstroms.

[0064] The reproducibility of the second profile is checked on a plurality of substrates 1 processed simultaneously in the etching tank 100 . Figure 6The curve in shows, in the form of a box plot, the results of the thickness gradient (in Angstroms) of the second profile compared to the results obtained when the introduction and removal speeds are standard (ie, greater than 25 cm / s).

[0065] The average distance between the two introduction points PII and PFI is clearly found to be 2.8 Å ( Figure 6 The "average" in ), standard deviation ( Figure 6 The “standard deviation” in ) is about 0.3 angstrom of etching gradient.

[0066] Applying the second non-uniform etch profile to a process such as Figure 3 The particular substrate 1 shown, exhibiting a non-uniform thickness (note the placement of the recess feature 22 at the final point of introduction PFI (and the initial point of removal PIS)), allows for an improvement in the uniformity of the thin layer 11. Specifically, the second profile allows the northern region (initially the thickest area) of the substrate 1 to be thinner relative to the southern region (initially the thinnest area). Thus, with the aid of the etching method according to the present invention, the initial non-uniformity of the thin layer 11, which was approximately 0.6 nm, is reduced to less than 0.4 nm.

[0067] Note that the introduction speed can also be limited between 25 cm / s and 0.1 cm / s to more finely tune the differential removal between the two introduction or removal endpoints.

[0068] The etching method according to this preferred embodiment can be easily integrated into the cleaning sequence of a chemical tank multi-wafer cleaning system (or "wet bench"). For example, it can be integrated into an ozone / SC1 / SC2 type sequence, replacing the SC1 step or inserting it between the ozone and SC1 steps. Direct integration of the etching method into the cleaning sequence is particularly advantageous because the rinsing and loading time for the batch of substrates 1 is shared and not multiplied by separate stages.

[0069] The etching method according to the invention is particularly suitable for correcting non-concentrically symmetrical inhomogeneities in a thin surface layer 11 of an SOI substrate 1 , said thin layer 11 and the underlying insulating layer 30 having a thickness of less than 50 nm.

[0070] It goes without saying that the present invention is not limited to the described embodiments and examples, and various implementations can be applied thereto without departing from the scope of the invention defined by the claims.

[0071] Although the invention has been described and illustrated with particular reference to preferred embodiments, it can of course be applied to thin layers 11 made of materials other than single-crystalline silicon (e.g. silicon oxide, etc.) and to etching bath solutions other than SC1 (e.g. hydrofluoric acid HF).

Claims

1. An etching method for etching a main surface (1a) of a substrate (1) comprising a surface thin layer (11), wherein the main surface (1a) corresponds to a free surface of the surface thin layer (11), the etching method comprising the following steps: The substrate (1) is immersed in an etching tank (100) so as to expose the main surface (1a) to an etchant, the substrate (1) being oriented relative to the etching tank (100) in such a manner as to compensate for the non-uniformity of the thickness of the surface thin layer (11): - when the substrate (1) is introduced (I) into the etching tank (100), the main surface (1a) is gradually immersed from the initial introduction point (PII) to the final introduction point (PFI) according to the introduction speed, and - when the substrate (1) is removed (S) from the etching tank (100), the main surface (1a) gradually emerges from the initial removal point (PIS) to the final removal point (PFS) according to the removal speed, The etching method is characterized in that: - the introduction speed is selected in such a way that the main surface (1a) is etched according to a first non-uniform profile between the initial introduction point (PII) and the final introduction point (PFI), and / or - the removal rate is selected in such a way that the main surface (1a) is etched according to a second non-uniform profile between the initial removal point (PIS) and the final removal point (PFS), Wherein, the introduction speed and / or the removal speed is between 25 cm / s and 0.1 cm / s.

2. The etching method according to claim 1, wherein - said first non-uniform profile defines a thickness variation greater than or equal to 0.15 nm etched on said main surface (1a) between said initial point of introduction (PII) and said final point of introduction (PFI), and / or - said second non-uniform profile defines a thickness variation greater than or equal to 0.15 nm etched on said main surface (1a) between said initial removal point (PIS) and said final removal point (PFS).

3. The etching method according to claim 2, wherein: - said first non-uniform profile defines a thickness variation greater than or equal to 0.2 nm etched on said main surface (1a) between said initial point of introduction (PII) and said final point of introduction (PFI), and / or - the second non-uniform profile defines a thickness variation greater than or equal to 0.2 nm etched on the main surface (1a) between the initial removal point (PIS) and the final removal point (PFS).

4. The etching method according to any one of claims 1 to 3, wherein The initial point of introduction (PII) corresponds to the final point of removal (PFS), and the final point of introduction (PFI) corresponds to the initial point of removal (PIS).

5. The etching method according to any one of claims 1 to 3, wherein The introduction speed and / or the removal speed is between 10 cm / s and 0.5 cm / s.

6. The etching method according to any one of claims 1 to 3, wherein When the substrate (1) is introduced into and removed from the etching tank (100), the introduction speed and / or the removal speed are respectively non-constant.

7. The etching method according to claim 1, wherein: The etching tank (100) contains an SC1 solution having ammonia, hydrogen peroxide and deionized water in a ratio of 1 / 2 / 20 respectively.

8. The etching method according to claim 1, wherein The substrate (1) is an SOI substrate, a surface thin layer (11) of which has a thickness of less than 50 nm and is arranged on an insulating layer (30), which is arranged on a carrier substrate (20) made of silicon.

9. The etching method according to claim 7 or 8, wherein: The surface thin layer (11) is made of single crystal silicon.

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