METHOD FOR MEASURING THE THICKNESS OF A SURFACE LAYER OF A SUBSTRATE

Abstract

The invention relates to a method for measuring at least one thickness parameter of a surface layer of an SOI substrate, comprising the following steps: a) measuring the thickness at a predefined number of first points, spatially distributed over the surface, resulting in a first series of values ​​including a first maximum and a first minimum; the thickness measurement being carried out by a first technique; b) creating a complete map of the surface of the surface layer by a second technique, different from the first technique, the map representing a physical characteristic of the surface layer correlated with the thickness; c) analyzing the map so as to identify whether there is a signature capable of representing a variation in thickness, greater than or equal to the difference between the first maximum and the first minimum, in a localized region of the surface layer;d) if a signature is identified, the thickness measurement at a plurality of second points positioned in the localized region, resulting in a second set of values ​​including a second maximum and / or a second minimum; the thickness measurement being performed by the first technique; e) the calculation of at least one thickness parameter, based on the first set of values ​​and, potentially, on the second set of values. Figure to be published with the abstract: No figure;

Description

Title of the invention: METHOD FOR MEASURING THE THICKNESS OF A SURFACE LAYER OF A SUBSTRATE FIELD OF INVENTION

[0001] The present invention relates to the field of semiconductors and in particular to SOI (Silicon on Insulator) substrates. The invention relates to a thickness measurement method applicable to a surface layer of an SOI substrate, in particular when the surface layer has an average thickness of less than 50nm: the method drastically improves the reliability of the thickness parameters (average, minimum, maximum, etc.) obtained, reliability being understood here as the fact that these parameters are representative of the surface layer over its entire surface.

[0002] TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] Increasingly, applications based on SOI substrates require very good uniformity in the thickness of the silicon surface layer. For example, in digital applications, the active layer of FDSOI ("Fully depleted SOI") substrates must exhibit very small variations in thickness because these affect the threshold voltage of the transistors fabricated in and / or on the active layer.

[0004] The specifications in terms of thickness and uniformity therefore become very aggressive: for a layer with a thickness typically less than 50nm, or even 25nm, a uniformity (corresponding to the difference between the maximum and minimum thickness of the layer) of less than Inm is expected.

[0005] In a manufacturing line, the thickness parameters (average thickness, uniformity, etc.) of a surface layer of FDSOI substrate are extracted from a limited number of measurement points (for example, 41 points), for understandable reasons of production rate. Even if the position of the measurement points on the surface of the surface layer is predefined to attempt to capture the thickness information most representative of the entire surface, it is not guaranteed that the minimum and maximum thicknesses will be recorded, as their location may vary from one substrate to another.

[0006] SUBJECT OF THE INVENTION

[0007] The present invention aims to improve the reliability of the thickness parameters (average, minimum, maximum, uniformity, etc.) obtained, reliability being understood here as the fact that these parameters are representative of the surface layer over its entire surface. It relates to a thickness measurement method applicable to a surface layer of an SOI substrate, in particular when the surface layer has an average thickness of less than 50 nm; the method implements a standard tool thickness measurement at a finite number of points, and uses information from mapping to identify signatures of potential thickness variations, and where appropriate, carry out additional thickness measurements in localized regions corresponding to these signatures.

[0008] BRIEF DESCRIPTION OF THE INVENTION

[0009] The invention relates to a method for measuring at least one thickness parameter of a surface layer of an SOI substrate, the surface layer having a surface extending along a principal plane, the method comprising the following steps:

[0010] a) the thickness measurement at a predefined number of first points, spatially distributed on the surface, resulting in a first series of values ​​comprising a first maximum and a first minimum; the thickness measurement being carried out by a first technique and the first points not covering the surface in its entirety;

[0011] b) the realization of a complete mapping of the surface of the surface layer by a second technique, different from the first technique, the mapping translating a physical characteristic of the surface layer correlated to the thickness;

[0012] c) the analysis of the mapping so as to identify whether there is a signature likely to reflect a variation in thickness, greater than or equal to the difference between the first maximum and the first minimum, in a localized region of the surface layer;

[0013] d) if a signature is identified, the thickness measurement at a second point or a plurality of second points positioned in the localized region, resulting in a second series of value(s) including a second maximum and / or a second minimum; the thickness measurement being carried out by the first technique;

[0014] e) the calculation of at least one thickness parameter, based on the first series of values ​​and, potentially, on the second series of value(s).

[0015] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: • the surface layer has an average thickness less than or equal to 50nm, less than or equal to 25nm, less than or equal to 20nm, or less than or equal to 15nm; • the second technique is reflectometry and the physical characteristic is thickness; • the second technique is based on a surface inspection by laser scattering in which a laser beam performs a scan of the surface of the surface layer, and the physical characteristic is the diffuse background noise (“haze”); • a signature corresponds to an abnormal variation in intensity of the diffuse background noise signal, and can appear in the form of a point pattern, a periodic wave, or other patterns; • the signature is identified on the map via an image recognition algorithm or by thresholding the intensity of the diffuse background noise signal; • at the end of step c), coordinates are assigned to the signature, in the main plan, by a piece of equipment - called second equipment - implementing the second technique, and said coordinates are communicated, prior to step d), to a first piece of equipment implementing the first technique; • the second points are positioned in the main plane according to distribution schemes established based on the shape of the signature; • the -at least one- thickness parameter calculated in step e) is uniformity; • several thickness parameters are calculated in step e), including a mean, and / or a median, and / or a standard deviation; • the first technique is spectral ellipsometry; • the predefined number of first points is 12, 41 or 625; • the second points are in the number of 2, 9, 16 or 25. BRIEF DESCRIPTION OF THE FIGURES

[0016] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures in which:

[0017] [Fig.1] Fig.1 presents the steps of a measurement method according to the present invention;

[0018] [Fig.2]

[0019] [Fig. 3] Figures 2 and 3 illustrate two examples of mapping obtained at the stage b) of the measurement method according to the invention ([Fig.2]: by reflectometry, first embodiment; [Fig.3]: by laser scattering inspection, second embodiment);

[0020] [Fig.4a]

[0021] [Fig.4b]

[0022] [Fig.4c]

[0023] [Fig.4d] Figures 4a, 4b, 4c, 4d illustrate four examples of maps obtained according to the second embodiment of step b), exhibiting particular signatures; they also illustrate four associated examples of the arrangement of the second measurement points in step d), in a measurement method according to the invention;

[0024] [Fig.5a]

[0025] [Fig.5b]

[0026] [Fig.5c]

[0027] [Fig.5d] Figures 5a, 5b, 5c, 5d illustrate an example of results obtained respectively at steps a), b), c), d) during the implementation of a method according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention relates to a method for measuring one or more thickness parameter(s) of a silicon surface layer of an SOI (silicon on insulator) substrate.

[0029] Thickness parameters include thickness uniformity, mean (average thickness), median, standard deviation, etc. These parameters are usually calculated from the thickness values ​​of the surface layer obtained at several measurement points. In particular, thickness uniformity can be determined by the difference between the maximum and minimum thicknesses measured.

[0030] A silicon dioxide (SOI) substrate is usually in the form of a circular wafer, with a diameter ranging from 150 mm to 450 mm, for example, typically 300 mm for FDSOI substrates. The silicon surface layer is deposited on a dielectric layer, which is itself deposited on a support substrate. The surface of the surface layer extends in a principal plane (x,y).

[0031] Advantageously, this method is applicable to an SOI substrate having a very thin surface layer, typically with an average thickness less than or equal to 50 nm, less than or equal to 25 nm, less than or equal to 20 nm, or even less than or equal to 15 nm. For such substrates, the buried dielectric layer of the SOI substrate is generally between 100 nm and 1000 nm.

[0032] Among the physical characteristics requiring compliance with specifications at the end of the manufacturing line, we of course include the thickness parameters of the surface layer of a SOL substrate. As mentioned in the introduction, it is not easy to guarantee that these thickness parameters are representative of the entire surface layer, because often a limited number of measurement points are implemented in the metrology recipes, in order to respect the production, grading and shipping rates.

[0033] To overcome this problem, the measurement method according to the invention comprises a succession of steps ([Fig.1]) which will now be described.

[0034] Step a) corresponds to measuring the thickness of the surface layer of the SOI substrate at a predefined number of points (called first points), spatially distributed on the surface of said layer. The spatial distribution of the first points in the plane The principal (x,y) can take different forms, more or less classic, for example, a cross, a spiral, a star, etc. The most peripheral points preferentially extend up to 3 mm from an edge (contour) of the SOI substrate, knowing that the surface layer generally extends at least up to 0.5 mm from said edge.

[0035] This thickness measurement is performed using a first technique, chosen for its accuracy and reliability on an SOI substrate. Preferably, the first technique is spectral ellipsometry. The F5X industrial equipment from KLA is a suitable example. The wavelength range is typically 240 nm - 750 nm, and the measurement accuracy is 0.03 nm for an SOI substrate with an average surface layer thickness of 50 nm or less.

[0036] The predefined number of first points may be, for example, 12, 41 or 625. A relatively small number of points is preferred to limit the loss of time at this metrology step a): the first points cannot thus cover the surface of the layer in its entirety.

[0037] Step a) results in a first series of thickness values ​​comprising a maximum (called the first maximum) and a minimum (called the first minimum). At this stage, an intermediate thickness uniformity can be calculated from the first maximum and minimum.

[0038] Step b) of the method corresponds to the creation of a complete map of the surface of the surface layer, said map representing a physical characteristic of the surface layer that is more or less correlated with, or representative of, its thickness. In other words, the physical characteristic is correlated with the specular reflectivity of the SOI substrate, a reflectivity which is itself a function of the thickness of the multilayer according to the Fresnel formalism.

[0039] By complete mapping, it is understood that the physical characteristic is measured over the entire surface of the surface layer, with a measurement grid (spacing between two consecutive measurement points) less than or equal to 5mm, 1mm, or even less than or equal to 500qm (for example 200qm, corresponding to the size of a pixel).

[0040] Step b) implements a second technique, different from the first technique. This second technique is advantageously chosen from among the techniques allowing a high measurement rate.

[0041] According to a first embodiment, the second technique is reflectometry, and the physical characteristic is thickness. For example, ACUMAP equipment from the company ADE can be used. It allows thickness measurement using grids with a spacing as small as 1 mm, i.e., more than 70,000 points, which provides a thickness map of the entire surface of the surface layer. Figure 2 illustrates an example of a thickness map obtained by reflectometry. tomography.

[0042] Although a measurement by reflectometry is less precise than a measurement by ellipsometry (as used in step a)), the measurement rate allows for the rapid acquisition of a map, which can, in the next step, be analyzed to possibly identify signatures of significant variations in thickness, potentially outside the areas measured in step a).

[0043] According to a second embodiment, the second technique corresponds to a surface inspection by laser scattering, in which a UV laser beam performs a scan of the surface of the surface layer.

[0044] It is common practice to inspect a SOI substrate by dark-field microscopy using an incident light beam projected onto an inspection point and scanning the exposed surface of the substrate to determine the level of defects. A light-collecting device and a detector allow the light scattered at the inspection point to be measured. Such an inspection can be performed, in particular, using inspection equipment such as the Surfscan™ SP1, SP2, SP3, or SP5 from KLA. When the inspected surface has a surface irregularity or a particle present at the inspection point, this leads to an increase in the scattered light signal. The measurement of this radiation constitutes a signature carrying information about the presence, position, and / or nature of a surface defect or particle at the inspection point.Scanning the entire surface layer with a laser beam allows for a complete defect map, which is also essential for grading a SOI substrate. The laser beam wavelength ranges from 200 nm to 500 nm, depending on the equipment. Preferably, it is in the ultraviolet (UV) or even deep ultraviolet (DUV) range, typically between 200 nm and 280 nm. For example, the SP5 equipment offers a 266 nm laser.

[0045] After scanning the surface of the surface layer with the laser beam, in addition to defect mapping, this type of equipment can also compile a map from the diffuse background noise signal (more commonly known as "haze"). The diffuse background noise corresponds to the diffuse reflectance of the measured substrate. An example of diffuse background noise mapping is shown in [Fig. 3], for a 300 mm diameter SOI substrate, with a surface layer of 12 nm average thickness and a buried dielectric layer (SiO2) of 25 nm average thickness.

[0046] The diffuse background noise signal is known to be representative of the surface roughness of the surface layer. In the case of a thin surface layer (typically less than or equal to 50 nm), particularly when the surface roughness is low and uniform (for example, less than 0.5 nm RMS on an AFM scan), this is especially true when the surface roughness is low and uniform (e.g., less than 0.5 nm RMS on an AFM scan). of 30x30um2, with a uniformity of less than 0.3mn), the diffuse background noise can be correlated to the thickness of the surface layer (Brun et al, “Defect inspection challenges and solutions for ultra-thin SOI” - 2012 SEMI Advanced Semiconductor Manufacturing Conference).

[0047] Indeed, diffuse reflectance is then very weakly affected by surface roughness and is correlated with specular reflectance, in the particular case where the surface layer is sufficiently transparent to the wavelength of the laser beam, so as to probe the entire thickness of the layer.

[0048] Thus, in the second embodiment, the physical characteristic imaged by the mapping in step b) is the diffuse background noise. This embodiment is particularly advantageous in that, since surface inspection by laser scattering is required to verify the defect level of an SOI substrate, performing the diffuse background noise mapping does not add metrology time to the SOI substrate production process.

[0049] Returning to the general description of the method according to the invention, the following step c) corresponds to the analysis of the mapping obtained in step b), so as to identify whether there is a signature capable of reflecting a variation in thickness, greater than or equal to the difference between the first maximum and the first minimum, in a localized region of the surface layer.

[0050] Taking as an example the mapping of [Fig. 2] (first embodiment of step b), a wave pattern SI is observed, extending globally over the entire surface of the surface layer. This SI signature is visible on the mapping (a pattern that stands out against the surrounding areas), and it is also characterized by a significant variation in thickness.

[0051] Taking as an example the mapping of [Fig. 3] (second embodiment of step b), at least one signature S2 is also observed, corresponding to an abnormal variation in the intensity of the diffuse background noise signal, and which takes the form of a periodic wave. Other signatures may take the form of a point pattern, an aperiodic wave, or other patterns.

[0052] Advantageously, the signature is identified on the map by means of an image recognition algorithm or by thresholding the intensity of the diffuse background noise signal.

[0053] An image recognition algorithm can be fed and trained on different types of signatures likely to be present on a diffuse background noise map; it can thus learn to recognize each of these signatures and classify them into predefined categories, taking into account criteria of size, shape, contrast, etc.

[0054] Signatures can alternatively be identified by applying a certain threshold (or several thresholds) to the intensity of the diffuse background noise signal, on parameters such as standard deviation, relative median or relative mean of the signal: an intensity detected beyond (or below) the threshold on the map then reflects a signature likely to correspond to a significant variation in thickness, in a localized region of the surface layer.

[0055] Figures 4a, 4b, 4c, and 4d show four examples of diffuse background noise maps on which different signatures are observed (circled by a white square). In [Fig. 4a], signature S3 corresponds to a point pattern, indicating a localized overthickness of the surface layer. In [Fig. 4b], signature S4 corresponds to a scratch originating from the edge of the plate and extending into the interior of the layer; it can generate a significant variation in the thickness of the surface layer. In [Fig. 4c], signature S5 corresponds to a large dark area, indicating an overthickness of the surface layer. Finally, in [Fig. 5d], signature S6 corresponds to a wave pattern characterized by variations in the thickness of the surface layer.

[0056] By way of example, when thresholds are applied to detect signatures: - a S3 signature of the point pattern type is identified when the relative median of the diffuse background noise signal collected in the "narrow" channel ("haze narrow") is greater than 0.025; - an S4 signature of the scratch type or other fine variations is identified when the standard deviation of the diffuse background noise signal collected in the "narrow" channel ("haze narrow") is greater than 0.04; - an S5 signature of the extended zone of overthickness type is identified when the relative median of the diffuse background noise signal collected in the "narrow" channel ("haze narrow") is less than 0.95; - a wave-type S6 signature is identified when the relative mean of the diffuse background noise signal collected in the "wide" channel ("haze wide") is less than -0.006.

[0057] As a reminder, in an SPx type device, the scattered light coming from the substrate is collected either by an elliptical mirror, for rays of incidence far from the normal (z) to the principal plane (x,y) of the layer (collector called "wide" or "wide" channel), or by a lens, for rays of incidence close to the normal (z) (collector called "narrow" or "narrow" channel).

[0058] Step b) and step c) can be carried out on one and the same equipment, which implements the second technique and includes an analysis module to identify potential signatures on the mapping.

[0059] Alternatively, step c) can be carried out by other equipment comprising the analysis module. The latter receives the diffuse background noise mapping files established in step b), applies signature detection (via an image recognition algorithm or by thresholding), and generates, if signatures of interest are present, a file containing the location of the signatures in question.

[0060] If no signature is detected on the map from step b), the calculation of the thickness parameter(s) (among which, for example, uniformity) can be made from the first series of values ​​obtained in step a). The absence of a signature confirms that these thickness values ​​are representative of the entire surface layer.

[0061] If at least one signature of interest is detected on the map, the method according to the invention provides a step d) of thickness measurement at a second point or at a plurality of second points positioned in the localized region (attached to the signature on the map). This thickness measurement step results in a second value (in the case of a single second measurement point) or a second series of values, including a second maximum and / or a second minimum. The measurement is performed using the first technique (for example, spectral ellipsometry).

[0062] Preferably, at the end of step c), coordinates are assigned to the signature in the main plane by the second piece of equipment (the one implementing the second technique). These coordinates are automatically communicated, prior to step d), to the first piece of equipment implementing the first technique, via the generation of a file containing the location of the signature.

[0063] In the first device, the second points are positioned in the principal plane according to distribution schemes established based on the shape and type of signature. In the examples illustrated in Figures 4a, 4b, 4c, and 4d, it can be seen that the distribution schemes for the second points can be in the form of a cross, centered on the coordinates of the center of the signature. The cross is larger than the signature so that at least the extreme points of the cross fall outside the signature: this ensures that the signature has been fully evaluated. In the figures, the size of the square surrounding all or part of the distribution scheme for the second points is indicated above the square.

[0064] In the example of [Fig.4d], a star or concentric distribution is preferred to capture possible thickness variations in all directions; a high density of second points at the center favors the detection of the thickness extremum, and the outer points, further away (typically at 10mm) capture the other thickness extremum of the wave pattern.

[0065] Preferably, the second points are 10, 16, or 25 in number, to avoid taking too much time to measure. Of course, any other number of second points is possible, depending on the size, shape, and type of signature.

[0066] Of course, if several signatures are identified on the mapping, each of the localized regions of the surface layer corresponding to these signatures will be evaluated in step d), by a local thickness measurement.

[0067] The measurement method according to the invention finally includes a step e) corresponding to the calculation of at least one thickness parameter of the surface layer, based on the first series of values ​​and, potentially on the second series of values, if step d) has taken place.

[0068] In the particular case where the -at least one- thickness parameter is uniformity, the first minimum and maximum are compared to the second minimum and maximum: the lowest of the minima and the highest of the maxima is then taken into consideration to determine the thickness uniformity of the layer.

[0069] Step e) may also include the calculation of other thickness parameters of interest, namely the mean, the median, the standard deviation, etc. These can be established from the first series of values, possibly supplemented by the second series of values. Examples

[0070] - SOI: average thickness of the surface silicon layer 12nm; thickness of the buried oxide (SiO2) layer 25nm; - Step a): measurement on the first 41 points ([Fig.5a]) by spectral ellipsometry (first equipment, first technique); obtaining a first series of thickness values: maximum 12.29nm, minimum 11.66nm; - Step b): mapping of diffuse background noise (second equipment, second technique) ([Fig.5b]); - Step c): analysis of the mapping (analysis module), identification of a point pattern type signature ([Fig.5c]) and transfer of the signature coordinates to the first equipment; - Step d): measurement on 9 second points, according to a cross distribution, centered on the signature, by spectral ellipsometry (first equipment, first technique), obtaining a second series of thickness values: maximum 14.47nm, minimum 13.15nm; - Step e): calculation of thickness parameters to obtain, in particular, a uniformity of 2.8 Inm based on the first and second sets of values; the uniformity based on the first set of values ​​was 0.64 nm. For a thickness uniformity specification of Inm, the measurement in step d) identified that the surface layer of this SOI did not meet the specification, whereas the first set of values ​​(step a)) alone would not have been sufficient to determine this.

[0071] The measurement method according to the present invention drastically improves the reliability of the thickness parameters (average, minimum, maximum, uniformity) taken into account for the grading of an SOI substrate. These parameters are representative of the surface layer over its entire surface, and the expected specifications are thus perfectly met.

[0072] Of course, the invention is not limited to the embodiments and examples described, and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

Demands

1. Method for measuring at least one thickness parameter of a surface layer of an SOI substrate, the surface layer having a surface extending along a principal plane (x,y), the method comprising the following steps: a) measuring the thickness at a predefined number of first points, spatially distributed over the surface, resulting in a first series of values ​​including a first maximum and a first minimum; the thickness measurement being carried out by a first technique and the first points not covering the entire surface; b) performing a complete mapping of the surface of the surface layer by a second technique, different from the first technique, the mapping representing a physical characteristic of the surface layer correlated with the thickness;c) the analysis of the mapping in order to identify whether there is a signature likely to reflect a variation in thickness, greater than or equal to the difference between the first maximum and the first minimum, in a localized region of the surface layer; d) if a signature is identified, the thickness measurement at a second point or a plurality of second points positioned in the localized region, resulting in a second series of value(s) including a second maximum and / or a second minimum; the thickness measurement being carried out by the first technique; e) the calculation of at least one thickness parameter, based on the first series of values ​​and, potentially, on the second series of value(s).

2. Measurement method according to the preceding claim, wherein the surface layer has an average thickness less than or equal to 50nm, less than or equal to 25nm, less than or equal to 20nm, or less than or equal to 15nm.

3. A measurement method according to any one of the preceding claims, wherein the second technique is reflectometry and the physical characteristic is thickness.

4. Measurement method according to claim 2, wherein the second technique is based on a laser scattering surface inspection in which a laser beam performs a scan of the surface of the surface layer, and the physical characteristic is the diffuse background noise (“haze”).

5. A measurement method according to the preceding claim, wherein a signature corresponds to an abnormal intensity variation of the diffuse background noise signal, and may be in the form of a point pattern, a periodic wave, or other patterns.

6. A measurement method according to one of the two preceding claims, wherein the signature is identified on the map by means of an image recognition algorithm or by thresholding the intensity of the diffuse background noise signal.

7. A measurement method according to any one of the preceding claims, wherein: - at the end of step c), coordinates are assigned to the signature, in the principal plane, by a piece of equipment - referred to as second equipment - implementing the second technique, and - said coordinates are communicated, prior to step d), to a first piece of equipment implementing the first technique.

8. Measurement method according to the preceding claim, wherein the second points are positioned in the principal plane according to distribution schemes established according to the shape of the signature.

9. A measurement method according to any one of the preceding claims, wherein the -at least one- thickness parameter calculated in step e) is uniformity.

10. A measurement method according to any one of the preceding claims, wherein several thickness parameters are calculated in step e), including a mean, and / or a median, and / or a standard deviation.

11. A measurement method according to any one of the preceding claims, wherein the first technique is spectral ellipsometry.

12. Measurement method according to any one of the preceding claims, wherein the predefined number of first points is 12, 41 or 625.

13. Measurement method according to any one of the preceding claims, wherein the second points are 2, 9, 16 or 25 in number.

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