Improved measurement method implementing a grazing incidence small-angle X-ray scattering technique – GISAXS; Instrumental system and associated computer program product.

Abstract

Improved measurement method employing a grazing incidence small-angle X-ray scattering technique – GISAXS; Instrumental system and associated computer program product. This method consists of: placing a line grating etched on a sample (10) in an X-ray beam so as to create a misalignment ( ) between the line grating and an incidence plane (PI); illuminating (130) the line grating at a given angle of incidence ( ); measuring (140) the quantity on the diffraction pattern obtained for a plurality of Bragg spots, the plurality of Bragg spots comprising one or more positive-order Bragg spots and one or more negative-order Bragg spots. Figure for the abstract: Figure 1

Description

Title of the invention: Improved measurement method implementing a technique by small-angle X-ray scattering at grazing incidence - GISAXS: Instrumental system and associated computer program product.

[0001] The present invention relates to a method of measurement by grazing-incidence small-angle X-ray scattering - GISAXS (for "grazing-incidence small-angle X-ray scattering")

[0002] To follow Moore's law and allow the increase in the number of chips etched per unit area, the size of the components (especially transistors) is constantly decreasing.

[0003] Their characteristic dimensions are now less than 10 nm.

[0004] The metrology required to visualize such components with subnanometer precision has led to the development of new techniques to circumvent the resolution limits of standard techniques, such as atomic force microscopy or scanning electron microscopy.

[0005] One promising technique is small-angle X-ray scattering, in particular grazing incidence small-angle X-ray scattering. Several studies at the synchrotron and with laboratory sources have been conducted to validate that the GISAXS technique can address this problem.

[0006] However, the implementation of this technique remains complex and the recording of measurements is tedious.

[0007] The object of the present invention is to propose a more efficient measurement method when implementing the GISAXS technique.

[0008] For this purpose, the invention relates to a measurement method implementing a technique by scattering X-rays at small angles at grazing incidence of a quantity relating to a periodic pattern forming a grid of lines, which is engraved on a sample, the method consisting of: placing the grid of lines in an X-ray beam so as to create a misalignment q> between the grid of lines and a plane of incidence; illuminating the grid of lines at a given angle of incidence a; measuring the quantity on the diffraction pattern obtained for a plurality of Bragg spots, the plurality of Bragg spots comprising one or more Bragg spots of positive order and one or more Bragg spots of negative order.

[0009] According to particular embodiments, the process comprises one or more of the following characteristics, taken individually or in all technically possible combinations:

[0010] - the misalignment is characterized by a misalignment angle, the misalignment angle alignment is selected between 0.01° and 1°, preferably equal to 0.1°.

[0011] - the measurement step is iterated for a succession of angles of incidence.

[0012] - being defined a set of angles of incidence extending between a minimum value and a maximum value, by predefined length step, the measurement step is iterated for each angle of incidence of said assembly.

[0013] - the measured quantity is selected from a position of the Bragg spot considered in reciprocal space and an intensity (I) of the Bragg task considered.

[0014] - following the measurement step, the measured quantities allow a curve to be fitted intensity which depends on a shape factor of the line network, in order to determine a probable profile of the line network.

[0015] The invention also relates to an instrumental system of the GISAXS type, comprising a computer and a detector connected to the computer, characterized in that the computer is suitably programmed so that the instrumental system implements the preceding process.

[0016] The invention also relates to a computer program product comprising software instructions which, when executed by the computer of an instrumental system conforming to the previous system, allows the latter to implement a process conforming to the previous process.

[0017] The invention and its advantages will be better understood upon reading the following detailed description of a particular embodiment, given solely by way of illustration and not limitation, this description being made with reference to the accompanying drawings in which:

[0018] [Fig-1] Fig. 1 is a schematic representation of an instrumental system for the implementation of the measurement method according to the invention, based on the implementation of a GISAXS technique;

[0019] [Fig.2] The [Fig.2] is an image obtained using the instrumental system of the [Fig.1], reported in reciprocal space, for a zero misalignment angle between the lines of a grid of lines and the plane of incidence;

[0020] [Fig.3] The [Fig.3] is a record of the measurements of the position coordinate qz as a function of the order of the Bragg task in the image of the [Fig.2] for different values ​​of the angle of incidence;

[0021] [Fig.4] The [Fig.4] is an image obtained using the instrumental system of the [Fig.1], referred in reciprocal space, for a non-zero misalignment angle between the lines of a line array and the plane of incidence;

[0022] [Fig. 5] Fig. 5 is a record of the measurements of the position coordinate qz as a function of the order of the Bragg task in the image of the figure for different values ​​of the angle of incidence;

[0023] [Fig.6] Fig.6 is a block representation of a mode of realization implementation of the measurement method according to the invention; and,

[0024] [Fig.7] Fig.7 illustrates the advantage of the measurement method in Fig.6 for two Line networks exhibit different form factors.

[0025] Fig. 1 represents an instrumental system 1 for implementing the measurement method according to the invention on a sample 10.

[0026] The measurement method according to the invention is based on a measurement technique by small angle X-ray scattering at grazing incidence - GIS AXS.

[0027] According to this technique, a source S illuminates the sample 10 and the light reflected by the sample 10 is imaged in an observation plane PO.

[0028] A frame x, y, z of the instrumental system is for example attached to the center O of the observation plane PO so that the normal direction to this plane is the direction x and the plane itself is defined by the directions y and z.

[0029] The sample 10 is, for example, a microelectronic component on the flat surface of which a target 12 is engraved, for example by implementing a lithography process. The target 12 has a network of lines.

[0030] A frame xO, yO, zO is attached to the center A of the target 12, such that the direction zO corresponds to the normal to the surface of the sample 10, that the direction xO corresponds to the direction of the lines of the grid of lines of the target 12, and that the direction yO is orthogonal to the directions zO and xO, i.e. orthogonal to the direction of the lines of the grid of lines.

[0031] The sample 10 is placed on a support 30 of the instrumental system. Preferably, and to simplify this description, the sample 10 is oriented so that the zO axis is parallel to the z axis of the instrumental system and the xOyO plane of the sample surface coincides with the xy plane. However, and more generally, there may be an angle between these two directions, which must then be taken into account during the analysis.

[0032] The X-ray source S emits an X-ray beam along a direction of incidence D on the target 12 of the sample 10.

[0033] The plane of incidence PI corresponds to the z, x plane. The y direction is therefore the normal direction to the plane of incidence PI.

[0034] A GISAXS measurement is carried out with a small angle of incidence a, between the direction x and D, for example of a few radians.

[0035] The support 30 allows the sample 10 to be rotated around the z-direction so as to modify an angle q> between the xO direction and the x direction, which is also the angle between the yO direction and the y direction. The angle q>, when not zero, causes a misalignment between the direction of the lines of the grid of lines and the plane of incidence PI.

[0036] The incident beam falls at point A of the sample. It is reflected and diffracted by the target 12 so as to form an image in the observation plane PO.

[0037] A detector 20 is placed in the observation plane PO. It is, for example, composed of a matrix of sensors arranged along the y direction and the z direction.

[0038] The intensity I at point B of the observation plane PO, measured by the sensor located at point B, depends on the angle 20 between the direction AO and the direction AB.

[0039] The detector 20 is connected to an electronic device, schematically represented in [Fig.1] by a cube bearing the reference 40.

[0040] The device 40 includes control electronics enabling the support 30 to be controlled so that it positions the sample 10 according to a setpoint value of the angle V.

[0041] The device 40 also includes acquisition electronics enabling suitable pre-processing to be carried out on the signals delivered by each of the sensors of the detector 20 and to digitize them.

[0042] The device 40 further comprises a computer for processing pre-processed and digitized signals. The computer is a computer comprising computing means, such as a processor, and storage means, such as memory. The memory stores, in particular, the instructions of computer programs, specifically a program whose execution enables the implementation of the measurement method according to the invention.

[0043] As known per se, the image in the observation plane PO is related to the spatial Fourier transform of the area illuminated by the incident beam.

[0044] In the context of grazing incidence measurement, in reciprocal space, the spatial frequency 0 is written, considering the spatial frequencies Qy and qz, respectively associated with the directions x, y and z of the frame linked to the observation plane PO:

[0045] - 22 [cos(df).cos(20) - cos(a)] (1)

[0046] q^ = 2s [cos (af), sin(2#)] (2)

[0047] q - [sin(ay) + sin(«)] (3)

[0048] where a is the angle of incidence of the X-rays on the sample, af is the angle of the ray reflected out of the xy plane, 20 is the angle of rotation around the z axis of the plane of incidence of the reflected ray and 2 is the wavelength.

[0049] Figure 2 represents a GIS AXS image reported in the reciprocal space Qy, ^z. This image is obtained for a line array perfectly aligned with the incident X-ray beam, i.e. with a zero angle P.

[0050] This image presents, in addition to a spot around the origin corresponding to the reflected beam, a diffraction pattern created by the target 12.

[0051] The diffraction pattern is composed of a plurality of spots, called Bragg spots,

[0052]

[0053]

[0054]

[0055]

[0056]

[0057]

[0058]

[0059]

[0060]

[0061]

[0062]

[0063]

[0064] arranged on an arc of a circle. Each spot is associated with a diffraction order. It is shown that a spot is actually the intersection of a diffraction rod, or Bragg rod, and the so-called Ewald sphere. The order of the Bragg spot is in fact the order of the associated Bragg rod. Changing the angle of incidence of the X-ray beam allows the Bragg rod to be probed. For the task located on the z-axis (qv — 0), we speak of principal order or zero order. Then, on the side of the positive Qy, when moving away from the z-axis, we successively observe the Bragg tasks associated with the positive Bragg orders 1, 2, 3, 4, 5, 6, 7, and 8. For negative 9y, when moving away from the z-axis, we observe the Bragg patches associated with the negative Bragg orders -1, -2, -3, -4, -5, -6, -7 and -8. We observe that the diffraction pattern is totally symmetrical with respect to the z-axis. The Bragg spot of order p corresponds by symmetry with respect to the z-axis to the Bragg spot of order-p. By changing the angle of incidence a, it is possible to probe the different Bragg orders. Figure 3 thus represents the measurements of the position in reciprocal space along the z, 9- direction, for the different Bragg spots of positive orders and for a succession of 21 values ​​of the angle of incidence a. According to the state of the art, the position of Bragg tasks is only measured for positive orders. Indeed, from a theoretical point of view, it is possible to calculate exactly the position in reciprocal space (qy, qz) of each diffraction spot. These calculations are presented, for example, in the article by M. Yan and A. Gibaud, “On the intersection of grating truncation rods with the Ewald sphere studied by grazing-incidence small-angle X-ray scattering”, Journal of Applied Crystallography (2007), 40, 1050-1055. When the grid of lines is perfectly aligned with the X-ray beam, the coordinates in reciprocal space Qy, of the Bragg task of order p are expressed according to the following equation: q_ = ^.cosasina. where P is the order of the Bragg task, b is the spatial period of repetition of the pattern in the sample structure (i.e. for a line array, it is the pitch - or distance - between two neighboring lines) along the y direction, 2 is the wavelength and a is the angle of incidence of the X-ray beam. For Qz, we observe that the order p appears in a squared term. Consequently, The sign of the order has no effect on the value of ¢-. The diffraction pattern is perfectly symmetrical with respect to the z-axis. The measurement of for the task of order p is equal to the measurement of for the order -p. Therefore, there is no additional information to be collected if, in addition to the positive orders, measurements are taken for the negative orders.

[0065] However, when the grid of lines is no longer aligned, the diffraction pattern is no longer symmetrical with respect to the z-axis.

[0066] Figure 4 represents a GISAXS image reported in the reciprocal space Qy, Qz. This image is obtained for a grid of lines deliberately misaligned with the incident X beam, i.e. with a non-zero angle V (in this case equal to 0.1°).

[0067] In this image, the spot around the origin still corresponds to the reflected beam, but the diffraction pattern created by the target 12 is now asymmetric.

[0068] It is composed of Bragg spots arranged on an arc of a circle, but these spots are no longer arranged symmetrically with respect to the z-axis.

[0069] Here again, from a theoretical point of view, it is possible to calculate exactly the position in the reciprocal space ^y, of each diffraction spot.

[0070] When the network of lines is no longer aligned, the coordinates in reciprocal space are expressed according to the following equation 2:

[0071] 2np£os <p Qy = -b lîT.sin ' acostpsirup 27T.sina.casa.cos <p

[0072] It is observed that the sign of the order p now influences the value of the position (and the value of the position Vy).

[0073] The measurement method according to the invention advantageously uses this asymmetry, by carrying out, for the same angle of incidence a, measurements of the position and / or intensity I of the Bragg spots for the positive orders but also for the negative orders of the diffraction figure.

[0074] Figure 5 thus represents the position measurements ^z for the different positive and negative orders and for a succession of 21 values ​​of the angle of incidence a. The measurements on the positive orders are indicated by a black circle and the measurements on the negative orders are indicated by a white circle (the order being given in absolute value).

[0075] Thus, for an experimental configuration (angle of incidence given) the measurements on the positive and negative orders make it possible to double the amount of information collected in roughly the same time period.

[0076] An embodiment of the measurement method according to the invention is presented to the [Fig.6].

[0077] The measurement method 100 begins with a step 110 consisting of orienting the target 12 with respect to the plane of incidence PI such that the grid of lines is misaligned with respect to the plane of incidence, i.e., there is a non-zero angle θ between the direction of the lines and the x direction. It should be emphasized that this misalignment angle does not have to be fixed rigorously. The aim is to create an asymmetry in the diffraction pattern.

[0078] Then, in a step 120, the angle of incidence a is initialized. For example, if one wants to explore a set of angles of incidence between a minimum angle a min and a maximum angle a max separated by a step size da, in step 120 the angle a is initialized to the value a min. For example, a min is equal to 0.5°, a max to 1.5° and the step size da is equal to 0.05°, i.e. 21 values ​​of the angle of incidence.

[0079] In step 130, the sample is illuminated by the X-ray beam so as to produce an image in the observation plane.

[0080] This image is analyzed in step 140.

[0081] More specifically, in a step 141, the order p is initialized to the value unit.

[0082] In step 142, for example, the position and intensity I of the Bragg task associated with the positive p order are measured, and then in step 143 the position and intensity I of the Bragg task associated with the negative -p order are measured.

[0083] In stable 144, a criterion is checked on the end of the analysis of the current image. For example, the number of orders considered is limited to a maximum order pmax.

[0084] If the criterion of step 144 is not met, the order p is incremented by one unit and the measurement steps are iterated.

[0085] If the criterion is met in step 144, the process 100 proceeds to step 150, which verifies a constraint on the value of the angle of incidence. If the maximum value amax has not been reached, the angle of incidence is increased, and the experimental setup is prepared with this new angle of incidence. Steps 130, which involve illuminating the sample with this new angle of incidence and analyzing the resulting image, are then repeated.

[0086] If at step 150, the angle of incidence has reached the upper bound of the interval to be explored, then the process ends.

[0087] For example, we obtain the record of [Fig.5].

[0088] In the embodiment of [Fig. 1], it is the orientation of the component in the X-ray beam that is modified to vary the angle of incidence. Alternatively, the component is fixed and the source is moved to vary the angle of incidence. The displacement of the source can be real or virtual using a suitable lens system.

[0089] Figure 7 illustrates the advantage of the method according to the invention. In an orthogonal plane zO, yO, the lines of the target line array present a profile. For example, on the left side of Figure 7, SI represents the transverse profile of a line of a first The network and S2 represent the transverse profile of a line from a second network. For example, these two networks are produced by lithography, but with different exposure times. After a lithography operation that etches the network of lines, and especially other circuit components, onto the sample, it is desirable to be able to determine whether the line profile is the SI profile or the S2 profile in order to verify the quality of the lithography.

[0090] The right-hand side of Figure 7 shows several graphs. Each graph represents, for a given order p, the intensity I (expressed in normalized units au) as a function of position for profile S1 (curve B1) and profile S2 (curve B2), respectively. The intensity curves B1 and B2 are obtained by means of numerical simulations from profiles S1 or S2.

[0091] Thus, multiplying the position and intensity measurements for positive and negative orders makes it possible to very quickly determine the profile of the lines actually engraved.

[0092] The measured quantities (position and intensity) are fitted to an intensity curve which depends on a shape factor of the line network and thus determine the most probable profile of the lines of the line network.

[0093] In this case, in [Fig. 7], since the measurements are better fitted to curve B1, it is concluded that the lines of target 12 have the profile SL

[0094] There may be little difference between the signal of two line networks having different profiles. With the implementation of the measurement method according to the invention, by performing twice as many measurement points, it is possible to describe the intensity curve of the Bragg rod more precisely and to reconstruct the profile of the line network.

[0095] The method of measuring the critical dimension of a periodic object (such as a line-of-sight grid) by breaking measurement symmetry allows for a doubling of efficiency and consequently halves the measurement time. The resulting time savings are crucial in lean manufacturing processes for microelectronics.

[0096] This measurement method is original because it goes against the common practice of the community using the GISAXS technique. According to the prior art, the line array is always aligned with the X-ray beam and, consequently, only a part (i.e., half) of the diffraction pattern is used for measurements and data collection.

[0097] It should be noted that the implementation of the measurement method according to the invention makes it possible to relax the constraint on the step size between two sets of measurements for two successive angles of incidence. Indeed, according to the prior art, in order to obtain a measurement density that allows the discrimination of two intensity curves, it is necessary to reduce the step size. That is, to achieve a small variation in the angle of incidence. However, it is not always easy to reconfigure the system to achieve a small change in the angle of incidence. With the invention, it is through measurement on the positive and negative orders that it is possible to achieve the required density, by relaxing the constraint on the value of which can be chosen to be slightly larger.

[0098] If, in the embodiment presented above, the target is a grating exhibiting periodicity only along the yO direction (thus forming lines extending along the xO direction), the target may also exhibit a periodic structure along the xO direction. If, in the embodiment presented above, the case of a target etched onto a sample has been presented, the method applies to any object exhibiting at least a local periodicity along one direction.

Claims

Demands

1. An improved method (100) for measuring a quantity relating to a periodic pattern forming a grid of lines etched on a sample (10), employing a grazing incidence small angle X-ray scattering (GISAXS) technique, consisting of: - placing (110) the grid of lines in an X-ray beam so as to create a misalignment (P) between the grid of lines and a plane of incidence (PI); and, given a set of angles of incidence extending between a minimum value and a maximum value (°Wy), in steps of predefined length (dd), for each angle of incidence of said set iterating the steps of: - illuminating (130) the grid of lines under said angle of incidence (a); - measuring (140) the quantity on the diffraction pattern obtained for a plurality of Bragg spots, the plurality of Bragg spots comprising one or more positive order Bragg spots and one or more negative order Bragg spots.

2. A measurement method according to claim 1, wherein the misalignment (^) is characterized by a misalignment angle, the misalignment angle being selected between 0.01° and 1°, preferably equal to 0.01°

3. V, 1. Measurement method according to any one of the preceding claims, wherein the measured quantity is selected from a position QJ of the Bragg task considered in reciprocal space and an intensity (I) of the Bragg task considered.

4. A measurement method according to any one of the preceding claims, wherein, following the measurement step, the measured quantities allow for the fitting of an intensity curve which depends on a shape factor of the line network, in order to determine a probable profile of the line network.

5. Instrumental system (1) of the grazing incidence small angle X-ray scattering type - (GISAXS), comprising a computer (40) and a detector (20) connected to the computer, characterized in that the computer is suitably programmed so that said instrumental system implements a measurement method according to any one of claims 1 to 4 to measure a quantity relating to a grid of lines etched on a sample (10).

6. Product computer program comprising software instructions which, when executed by a computer of an instrument system according to claim 5, enables the latter to implement a method according to any one of claims 1 to 4.