METHOD FOR INSPECTING THE SURFACE OF AN OBJECT

Abstract

The invention relates to a method implementing a device (1) for inspecting a surface (S) of an object.The device comprises a polychromatic light source (20) for projecting an inspection beam onto the surface (S); a confocal mask (6a, 6b) intercepting the inspection beam and a beam reflected by the surface, having a plurality of chromatic filtering apertures; a chromatic system (3) spatially spreading the focusing of the inspection beam according to focusing planes along an optical axis (AO) in a depth of field, intercepting the reflected beam to project it onto a detection plane (P) conjugate of the focusing planes; a movable support (4) for positioning the surface (S) in the depth of field; a time-integrated image sensor (5) synchronized with the movement of the surface, the image sensor (5) comprising a matrix of photodetectors arranged in the detection plane (P), the chromatic filtering apertures of the confocal mask illuminating a portion of the photodetectors. Fig. 1.

Description

Title of the invention: METHOD for inspecting a surface of an object FIELD OF THE INVENTION

[0001] The present invention relates to a device and a method for inspecting a surface of an object. A device and a method according to the invention allow the inspection of the surface of the object when it includes large variations in height and allows the generation at high rates of intensity images representative of the reflectivity of this surface with an extended depth of field and a high lateral resolution. In general, the invention relates to the field of imaging for the inspection and control of objects, such as substrates for the semiconductor or integrated optics industry (semiconductor wafers or substrates in all forms, during or at the end of the manufacture of semiconductor devices), or other objects in the fields of mechanics or the manufacturing industry for example (micro-mechanisms, watch parts or assemblies, assembled systems). TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] In the field of the semiconductor industry, the increase in production rates of semiconductor device wafers and the reduction in the size of the patterns create a growing need for high-speed imaging inspection. The methods for imaging these wafers must therefore allow the rapid localization of defects, structures and patterns over the entire surface of the wafers. Furthermore, due to the curvature of the wafers in particular, or due to the height of the structures to be imaged (wafer edges, etc.), it is necessary to be able to produce clear images in an area of ​​extended depth of field. In other fields, such as micromechanics or watchmaking, the objects to be inspected may have a relief or significant differences in height, also requiring extended depths of field.

[0003] Optical imaging systems such as conventional microscopes are known. These systems are generally limited in depth of field: the optical diffraction limit makes it very difficult to have both high magnification and a large depth of field. This is particularly disadvantageous when inspecting distant surfaces or structures with significant heights or depths. This problem is generally solved by keeping the objective at the desired distance from the measured surface, using an autofocus type system. This results in reduced movement speeds and more complex systems, or even the need to perform several passes to acquire the images.

[0004] Document US 8,654,324 implements a method for increasing the depth of field by exploiting the chromatic dispersion of a chromatic objective. The light transmitted through a slit is projected onto an object through a chromatic objective. The light reflected by the inspected surface is returned to the chromatic objective and then transmitted through a slit placed in front of a linear detector. The chromatic objective used in this configuration makes it possible to increase the measurement depth of field by exploiting the fact that different wavelengths are focused at different distances on the object. However, this system is not suitable for the formation of intensity images in two dimensions and with high lateral resolution. This is due to its slit configuration which does not meet the criteria for confocal detection in one dimension and thus generates crosstalk between measurement channels.

[0005] Document US 9,739,600 B1 describes a system allowing the localization of patterns in two dimensions with an extended depth of field obtained by a chromatic objective. The profile or the distance of the identified structures can be measured by the same instrument while respecting the criteria of confocal detection, with a minimum of cross-talk between measurement channels. The acquisition speed of the system is however directly limited by the quantity of light collected by the photodetector and especially by the number of measurement channels that it is possible to implement.

[0006] More particularly, the inspection rates depend on the optimization of the filling factor of the measurement points on the inspected wafers and defining the density of points measured at each instant. The filling factor is defined as the ratio between the diameter of the measurement points (diameter of the light beam projected onto the wafer at the focusing point) and the separation between two measurement points. Thus, the use of optical fibers or integrated optical circuits poses a physical limitation on the possibility of bringing the measurement paths closer together.

[0007] One of the objectives of the invention is to propose a device and a method for rapidly imaging a surface of an object over an extended depth of field.

[0008] Another objective of the invention is to propose such a device and a method allowing a high density of measurement points (high spatial resolution) while minimizing light coupling (cross-talk) between the detection paths.

[0009] Another objective of the invention is to enable localization and inspection of structures and patterns on a surface of an object such as a semiconductor substrate. The patterns may in particular be solder bumps arranged on conductive pads of the electronic chips of a wafer. BRIEF DESCRIPTION OF THE INVENTION

[0010] With a view to achieving one of these aims, the invention proposes a method of inspecting a surface of an object, the method comprising: - a first setting-up step during which the object is placed on a mobile support of an inspection device so as to position the surface in a depth of field of a chromatic system of this device, the chromatic system spatially spreading the focusing of a polychromatic inspection beam according to focusing planes in said depth of field arranged along an optical axis, and intercepting a beam reflected by the surface to project it onto a matrix of photodetectors of a time-delayed integration image sensor arranged in a detection plane, conjugate of the focusing planes; - a second step of activating at least one polychromatic light source to project the inspection beam onto the surface and thus form the reflected beam, at least one confocal mask having a plurality of chromatic filtering openings intercepting the inspection beam and the reflected beam, the chromatic filtering openings of the confocal mask making it possible to illuminate a portion of the photodetectors; - a third survey step during which the mobile support is controlled to move the surface relative to the chromatic system at least in an inspection direction perpendicular to the optical axis and to travel a measurement trajectory, and during which the image sensor is controlled to synchronize it with the movement of the surface; - a fourth step of acquiring the data established by the image sensor and preparing an image formed of pixels respectively associated with measurement points on the surface of the object, each pixel reflecting the intensity of the radiation reflected by the measurement point on the surface of the object.

[0011] According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination:

[0012] - the object is a chip wafer, the wafer being provided with a plurality of solder bumps; - the weld bumps are arranged in a line, and the path is at least partly parallel or perpendicular to the weld bump lines; - the trajectory is formed from a plurality of passages presenting different altitudes; - the third and / or the fourth step are implemented by a control unit of an inspection device; - the inspection method comprises a step of analyzing the image to identify the arrangement of projecting elements of the surface; - the analysis stage leads to determining whether the arrangement of salient elements conforms to an expected arrangement; the image sensor is a color or multispectral image sensor providing color or spectral information, and the method comprises a step of converting the provided color or spectral information into distance information to form a 3D image of the surface of the object; the matrix of photodetectors is arranged in the detection plane and arranged in a plurality of rows and columns, the charges generated by the photodetectors being able to be moved from one row to another, in a column direction synchronized with the movement in the inspection direction; the chromatic filtering openings are arranged on the confocal mask to illuminate a plurality of photodetectors arranged on a column of the matrix corresponding to a plurality of measurement points arranged in the inspection direction; the chromatic filtering apertures are distributed in a complementary manner over at least two lines of the confocal mask to illuminate a plurality of photodetectors corresponding to a plurality of measurement points arranged along at least two lines arranged in a direction perpendicular to the inspection direction; the confocal mask comprises a plurality of groups of at least two lines comprising chromatic filtering apertures distributed in a complementary manner on said at least two lines; the inspection method comprises a confocal illumination mask arranged in the inspection beam and a confocal detection mask arranged in the reflected beam, the confocal illumination mask and the confocal detection mask being respectively placed in a conjugate plane of the focusing planes with respect to the chromatic system; the inspection method comprises a single confocal mask, arranged in a jointly conjugate plane of the detection plane and the focusing planes with respect to the chromatic system; the polychromatic light source comprises a halogen lamp or a strip of light-emitting diodes, associated with a diffusion device to homogenize the intensity of the inspection beam; the polychromatic light source has a line spectrum, having a determined number of emission wavelengths or wavelength bands; the color system includes a color lens; the color system includes a separating element; the photodetectors are equipped with a color filter or a spectral filter; the chromatic filtering apertures are arranged to illuminate each a plurality of photodetectors equipped with filters of different colors or spectral bands. Brief description of the drawings

[0013] Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the appended figures in which:

[0014] [Fig-1] [Fig.l] represents an inspection device conforming to a mode of implementation in implementation of the invention;

[0015] [Fig.2] [Fig.2] represents a schematic diagram of the chromatic operation confocal implemented by the device of [Fig.l];

[0016] [Fig.3] [Fig.3] represents a front view of a detection mask illuminated by a reflected beam;

[0017] [Fig.4] [Fig.4] represents an aperture arrangement on a confocal mask in alignment to a photodetection matrix;

[0018] [Fig.5] [Fig.5] shows two inspection fields on the surface of a surface to be inspect. DETAILED DESCRIPTION OF THE INVENTION

[0019] For the sake of simplification of the description to come, the same references are used for identical elements or elements providing the same function in the state of the art or in the different modes of implementation of the device described.

[0020] [Fig.l] represents an embodiment of an inspection device according to the present invention.

[0021] Generally speaking, the inspection device 1 projects an inspection light beam onto a surface S of an inspected object. The beam is reflected on this surface S to project onto a detection plane P of an image sensor 5.

[0022] The inspected object, such as a semiconductor substrate, is arranged on a movable support 4 of the inspection device 1, for example a table whose movement can be controlled. The movable support 4 makes it possible to move the surface in at least one inspection direction I under the inspection beam.

[0023] The inspection device 1 comprises a lighting system 2 comprising a polychromatic light source 20 for forming the inspection beam. The source 20 therefore emits several wavelengths which may for example be included in the visible spectrum, in a wavelength interval between 380 nm and 700 nm, or in the infrared beyond 700 nm.

[0024] Generally speaking, the wavelengths composing the emission spectrum of the source 20 are chosen according to the nature of the object inspected and the layers which are likely to cover it. The wavelengths of the polychromatic source 20 will be chosen so that the inspection beam is reflected by the surface S of the object to be inspected. Furthermore, when a covering layer is present, the wavelengths of the polychromatic source 20 can be chosen so that they are included in the transparency spectrum of this covering layer.

[0025] By way of example, the polychromatic light source 20 may be formed from a halogen lamp or a strip of light-emitting diodes.

[0026] It may be a source emitting a broad spectrum to emit so-called "white" light, or a source having a spectrum in lines, and therefore having a determined number of inspection wavelengths or bands of inspection wavelengths. Each line may be provided by a laser source or by a source covering a narrow band of wavelengths.

[0027] It is generally sought to form an inspection beam of significant transverse dimension so as to illuminate a large portion of the inspected surface S, designated "inspection field" in the remainder of this description. In an application in the semiconductor field, the inspection field may have a dimension of between a few hundred microns and a few millimeters on each side. For this purpose, it is possible to provide, as is the case in the embodiment shown in [Fig.l], a diffusion device 21 to homogenize the intensity of the inspection light beam produced by the lighting system 2. The diffusion device 21 may for example be made of frosted glass or an optical guide element ensuring a mixture of the propagation modes of the radiation produced by the polychromatic source 20 so as to make the intensity of the inspection beam homogeneous.

[0028] Optical fibers 22 or any other form of guide for light radiation can also be provided, to conduct the light beam produced by the source 20 to the injection zone of the inspection beam in the device 1, here occupied by the diffusion device 21.

[0029] Of course, the lighting system 20 is not limited to that described in this embodiment. It may for example be envisaged that it is composed of a super continuum polychromatic light source whose radiation is guided by at least one optical guide to the injection zone.

[0030] Continuing with the description of [Fig.l], the inspection device 1 also comprises a chromatic system 3 arranged on the optical path of the inspection beam and the reflected beam. The chromatic system 3 generates a chromatic dispersion of the light passing through it and thus spatially spreads the focusing of the polychromatic inspection beam, depending on the wavelengths present, according to focusing planes distributed along an optical axis AO in a depth of field of the chromatic system 3 extended relative to the depth of field obtained for an individual wavelength.

[0031] For this purpose, the chromatic system 3 comprises a chromatic objective 3a leading to focusing the different wavelengths of the inspection beam, over its entire transverse extent, at different distances from the objective. The chromatic objective 3 can be produced in multiple ways, for example by means of a lens or a combination of lenses. These lenses can intercept the entire transverse extent of the inspection beam, or be respectively dedicated to portions of this beam constituting measurement paths. They can also be diffraction lenses or holographic devices. They can also be metallenses, i.e. reflective or transmissive optical elements having structured reflection or transmission surfaces.The patterns constituting this surface structuring have dimensions smaller than the wavelengths of the inspection light beam and affect the phase of the radiation of this beam and therefore its shape and propagation. The surface patterns of the optical elements can be manufactured by known methods of photolithography and etching.

[0032] Whatever the nature of the chromatic objective 3a, it is chosen to present a significant chromatic aberration, leading to spatially spreading the inspection beam, according to the wavelengths which compose it, according to focusing planes arranged in the depth of field of the chromatic system 3. This depth of field can for example extend over a distance of 100 to 200 microns.

[0033] During a measurement, the surface S of the inspected object is arranged in the depth of field of the chromatic system 3. For this purpose, it can be provided that the mobile support 4 can adjust the position of the surface S by moving the inspected object in the direction of the axis AO of the chromatic system 3, so as to maintain the surface S of the object in the depth of field. This characteristic is particularly interesting when the surface S has significant reliefs in elevation or depression.

[0034] Consequently, and advantageously, the mobile support 4 may consist of a table provided with means capable of controlling its movement in a plane perpendicular to the inspection beam in order to move the surface in the inspection direction I, and complementary means ensuring movement in a direction perpendicular to this plane in order to maintain the surface S in the depth of field of the chromatic system 3. However, to the extent that the depth of field of the chromatic system 3 is extended, the constraints for maintaining the surface S there are relaxed compared to an achromatic system.

[0035] The spatial displacement of the surface S during a measurement sequence can be controlled by an electronic control unit 7. This can also collect the trajectory carried out during the measurement sequence in order to be able to reconstruct a faithful image of the inspected surface S.

[0036] Continuing the description of the mode of implementation shown in [Fig.l], the chromatic system 3 is also provided with a splitter element 3b, for example a splitter cube or a reflective blade, to intercept the beam reflected by the surface S and project it onto a detection plane P of an image sensor 5. The splitter element 3b is arranged on the optical path of the reflected beam, downstream of the chromatic optics 3a, so that the reflected beam passes through this objective 3a before being projected onto the detection plane P.

[0037] Other optical parts of the chromatic system 3 may be placed on the optical path between the separating element 3b and the detection plane of the image sensor 5, but in all cases, the detection plane P is arranged in a conjugate plane, with respect to the chromatic system 3, of the focusing planes.

[0038] According to the invention, the image sensor 5 is a time-delayed integration image sensor (referred to as "image sensor" in the remainder of this description for simplicity). The detailed operation of such a sensor is for example described in the document EP2088763. It comprises a matrix of photodetectors arranged in a plurality of rows and columns. The charges generated by the photodetectors are moved from one row to another, in a column direction synchronously with the movement of the imaged object. The last row of the matrix is ​​moved into a shift register, or any other form of electronic component making it possible to extract the charges from this row, in order to constitute an image row of the object.

[0039] In the inspection device 1, the photodetector matrix of the image sensor 5 is arranged in the detection plane P onto which the reflected beam is projected. In other words, the image sensor 5 is arranged in the inspection device 1 to image the inspection field. Such a matrix may have a very large number of rows and columns, for example more than 10 thousand columns and several hundred rows.

[0040] The image sensor 5 is connected to the electronic control unit 7 in order to synchronize it with the movement of the surface S along the inspection direction I, and to repeatedly recover the charge lines extracted from the matrix of photodetectors at the end of the columns. In this configuration, and in the absence of any masking, a point of the surface S which moves along the inspection direction I is imaged successively on the photodetectors of a column of the matrix, i.e. on successive lines of the column. Simultaneously, the electric charge generated by a photodetector is moved synchronously from one line to another, along a column direction, so that the charge accumulated at the end of this scan, at the end of the column, is representative of the light reflected by the point of the surface S throughout the duration of its movement, in the conjugate plane of the image sensor 5, along the corresponding column of the sensor in the inspection direction I.

[0041] The inspection device shown in [Fig. 1] also comprises two confocal masks 6a, 6b arranged in conjugate planes of the surface S of the object with respect to the chromatic system 3, and respectively placed in the optical path of the inspection beam and the reflected beam.

[0042] A first confocal lighting mask 6a is here arranged on the diffusion device 21. The mask has a plurality of lighting openings. These openings can be crossed by the inspection beam coming from the lighting system, and the confocal lighting mask blocks the beam outside these openings. The inspection beam is therefore made up of a plurality of lighting paths defined by the lighting openings, projecting onto the surface S at a plurality of measurement points distributed in the inspection field of the device 1, according to focusing planes as a function of the wavelengths in the depth of field of the chromatic system 3.

[0043] A second confocal detection mask 6b is here arranged on the detection plane of the image sensor 5. Similar to the first confocal illumination mask 6a, the confocal detection mask 6b comprises a plurality of chromatic filtering apertures (or chromatic confocal), conjugated with the illumination apertures of the confocal illumination mask 6a. More precisely, the illumination apertures and the chromatic filtering apertures are conjugated with the same measurement points by the chromatic system 3.

[0044] The confocal detection mask 6b and the image sensor 5 are arranged relative to each other so that the chromatic filtering openings of the confocal mask 6b are respectively placed opposite a photodetector or a group of photodetectors and illuminate a portion of the photodetectors of the matrix.

[0045] For this, the confocal detection mask 6b can be plated, glued, or produced directly on the surface of the image sensor 5. It can in particular be produced by selective deposition of layers of dielectric or metallic materials on the surface of the sensor 5.

[0046] The confocal detection mask 6b can also be produced in the form of an element distinct from the sensor 5, positioned in a conjugate optical plane of the sensor by a lens or an imaging system.

[0047] In the inspection device 1 thus configured, the illumination paths of the inspection beam pass through the chromatic objective 3a. The beam is focused, according to the different wavelengths which constitute it, in focusing planes distributed over the very wide depth of field by the effect of the chromatic objective 3a. The radiation of each illumination path is reflected by the inspected surface S, arranged in the depth of field, at the measuring points, passes again through the chromatic objective 3a, and is projected onto the confocal detection mask 6b via the splitter element 3b.

[0048] [Fig. 2] thus shows a schematic diagram of the confocal chromatic operation implemented by the device of [Fig. 1]. In this figure, the illumination mask 6a has an illumination aperture defining an inspection beam here comprising a single illumination path. By the effect of the chromatic objective 3a arranged on its optical path, the inspection beam focuses, depending on the optical wavelengths present, on an extended depth of field. [Fig. 2] shows a first wavelength Xo of the beam which focuses in a focusing plane PfO and a second wavelength Xi of the beam which focuses on a second focusing plane Pfl, quite distinct from the first plane.

[0049] The surface S to be inspected was placed at the first focusing plane PfO, so that the illumination aperture of the illumination mask is imaged there for the first wavelength. The light at the first wavelength Xo coming from the illumination aperture is therefore reflected on the surface S to focus on the detection plane P of the image sensor 5, in the chromatic filtering aperture of the detection mask 6b. Thus, this light at the first wavelength Xo reaches the detection plane P in all or almost all of its intensity through the mask aperture.

[0050] On the contrary, the light at the second wavelength Xi is not perfectly focused on the first focusing plane PfO in which the surface S resides. As a result, this light is reflected diffusely, up to the detection mask 6b which blocks it largely, so that little of its intensity reaches the detection plane P. [Fig. 3] shows a front view of the detection mask 6b, which here has 12 chromatic filtering apertures O arranged in 4 columns and 3 rows. The mask 6b is illuminated by the reflected beam in a setup similar to that shown in [Fig. 2]. [Fig. 3] also shows the outline of the luminous halo H corresponding to the projection onto the mask of radiation at wavelengths reflected outside their focusing planes on the inspected surface S.

[0051] To limit the coupling phenomenon between different detection paths, it is important that the openings of the masks 6a, 6b are sufficiently far from each other, so that the diffuse halo associated with an opening does not cover a neighboring opening with too much intensity.

[0052] For example, the centers of two adjacent openings on the illumination mask 6a or on the detection mask 6b may be separated by a distance greater than twice the dimension of these openings (their diameters if these openings are circular).

[0053] However, moving the openings of the masks 6a, 6b away from each other leads to a reduction in the filling factor of the measurement points in the inspection field. As noted in the introduction, a reduction in the filling factor affects the inspection rate and the density of the measurement points.

[0054] To solve this problem, the openings of the lighting and detection masks are cleverly arranged with respect to the photodetectors of the image sensor 5.

[0055] Thus, [Fig. 4] shows a part of the photodetector matrix in the form of a grid G. Each box of the grid symbolizes a photodetector or a group of photodetectors. The part of the matrix shown is composed of 5 columns (referenced C1 to C5) and 5 rows (referenced L1 to L5). The openings of the masks 6a, 6b are represented by circles, arranged here in alignment with the photodetectors of the camera. These openings are capable of passing the reflected beam which is then projected onto the exposed photodetectors to be measured by the image sensor 5. It should be noted that these are indifferently the openings of the illumination mask 6a or of the detection mask 6b, these two masks being optically conjugated to each other when the photodetectors are exposed.

[0056] In this arrangement, the chromatic filtering apertures are arranged on the confocal mask (6a, 6b) to respectively illuminate a plurality of photodetectors, or a plurality of groups of photodetectors, arranged on a column (C1-C5) of the matrix. The image sensor 5 is synchronized with the movement of the surface to be inspected so that the plurality of photodetectors of a column under the apertures corresponds to a plurality of measurement points arranged along the inspection direction (I). When the surface S is moved along the inspection direction I, the photodetectors of the column under the apertures of the mask are successively exposed to the light coming from a measurement point.At the end of the column, a charge is recovered corresponding to the quantity of light reflected by the measuring point during an extended exposure time, corresponding to the exposure time of each photodetector multiplied by the number of openings in the column. In this way, a high-intensity image of the inspected surface can be produced and / or the inspection rate can be increased.

[0057] In this arrangement also, the chromatic filtering apertures are distributed on the confocal mask 6b in a complementary manner over at least two lines, that is to say for example that for a given column, if a line does not comprise a chromatic filtering aperture, another line comprises one. In particular, such a complementary distribution makes it possible to ensure that, for a group of lines which each comprise a chromatic filtering aperture, for a sub-part of the columns, each column comprises the same number of chromatic filtering apertures (for example one) for the group of lines. thus exposes a plurality of photodetectors equally distributed in a complementary manner on at least two rows of the matrix. In the example of [Fig.4], the photodetectors of rows L1 and L2, and of rows L3 and L4 are thus arranged in a complementary manner on these two rows. In other words, two contiguous photodetectors in a row or column are not simultaneously exposed under an opening in the mask. In this way, the openings are sufficiently spaced apart from each other to limit parasitic coupling between several detection paths. The image sensor 5 is synchronized with the movement of the surface S to be inspected so that the photodetectors distributed on the rows correspond to a plurality of measuring points arranged similarly along lines arranged in a direction perpendicular to the inspection direction (I).Due to the complementary distribution of the chromatic filtering apertures between the lines, when the surface S is moved along the inspection direction I the measurement points imaged on the detector combine very densely in the inspection field.

[0058] Thus, [Fig. 5] shows a first inspection field C11 as well as the measurement points (in dotted lines) imaged on the image sensor 5 of this inspection field. The surface has been moved in the inspection direction and a second inspection field CI2 as well as the measurement points in this field (in solid lines) have been shown. The distribution of the openings over several complementary lines allows dense measurements to be taken in the inspection field as the object moves.

[0059] Of course, the openings can be distributed in a complementary manner over a greater number of rows, which makes it possible to space these openings even further apart from each other and further limit the coupling phenomenon. For a given dimension of the photodetector matrix, this amounts to placing fewer photodetectors on a column, and therefore affecting the intensity quality of the image.

[0060] Advantageously, the confocal mask (6a, 6b) may comprise a plurality of groups of lines with a complementary distribution of the chromatic filtering apertures. These groups of lines may comprise, for example, an identical distribution of chromatic filtering apertures. Thus, each measurement point of the object is imaged on the detector in a plurality of chromatic filtering apertures, sequentially, which makes it possible to accumulate more intensity or charges and to fully exploit the number of lines of the photodetector matrix.

[0061] The device as described makes it possible to obtain intensity images, or gray levels, of the surface of the object, or in other words of its reflectivity.

[0062] Since the chromatic system performs a coding of the height of the object in wavelength, it is also possible to obtain depth information. This requires determining the wavelength of the radiation detected by the photo detectors. This can be implemented by using a color image sensor, thus returning color information in addition to the intensity information of the detected radiation. By "color image sensor" is meant any image sensor capable of associating wavelength information with the intensity of the radiation received at a photodetector. These sensors are therefore capable of forming color images, for example by coding the color information in the form of RGB pixels, as is very conventional, in which each pixel is associated with the light intensity detected in red, green and blue. But more generally, the color image sensors according to the present description are capable of providing hyperspectral images, a pixel of such an image being associated with the light intensity detected in one band among a plurality of wavelength bands, forming spectral decomposition bands of the received radiation.This plurality of bands can be significant, for example greater than 50, 100, and generally between 50 and 200. Thus, pixels or groups of pixels of the detector are respectively sensitive to light in a given spectral band. In this way, it is possible to cover and decompose a wide useful spectrum, this useful spectrum of course corresponding to the emission spectrum of the polychromatic source 20.

[0063] This approach can be implemented in different ways.

[0064] According to a first approach, a plurality of photodetectors are arranged under the mask openings, respectively equipped with color filters, for example RGB, or equipped with interference filters only allowing light to pass in a specific spectral band. Each measurement point is then characterized by the intensity measurement in each of the spectra defined by these filters.

[0065] According to another approach, the photodetector (or photodetectors) under an opening is provided with a single color or interference filter, but the nature of this filter varies from one column to another, along the line. The radiation emitted by adjacent measurement points of the surface S, corresponding to adjacent columns of the detector, is therefore detected by photodetectors respectively dedicated to a color.

[0066] According to a further variant, the image sensor has a plurality of TDI photodetector matrices as described previously, and each matrix is ​​associated with a single color (associated with a single color or interference filter, to use the terminology of the preceding paragraphs). Advantageously, the columns of the matrices are aligned with each other, so that the same measurement point of the surface S is successively imaged by the photodetectors of each of the matrices. Each matrix is ​​associated with a confocal mask 6b as described previously. In this way, it is possible to very precisely associate a single measurement point of the surface S with a plurality of color information.

[0067] The color information, even coarse, obtained for example using an RGB or interference type filter, can advantageously be used by the electronic control unit 7 to estimate the elevation of the measurement points, and more specifically the distance separating the measurement point from a reference plane of the device. The control unit 7 can therefore estimate an average distance separating the surface S from this reference plane. It can control the movement along the optical axis AO of the mobile support 4 on this average distance estimate to maintain the inspection surface S in the depth of field of the chromatic system 3 while moving this surface S along the inspection direction I, during a measurement sequence.

[0068] The speed of movement of the surface S, according to the inspection direction, can reach, for example, a value of the order of 100 mm / second.

[0069] An example of application of the inspection device 1 which has just been described will now be described, this example forming a particularly interesting mode of implementation of the invention.

[0070] The object to be inspected in this application is for example a semiconductor wafer, for example based on silicon, or any other material or assembly of materials whose properties can be exploited to form integrated electronic components. The wafer has undergone prior treatments of a conventional manufacturing process in the field of microelectronics and / or microelectromechanical systems, so as to form finished or semi-finished integrated chips on the surface of this wafer. The chips have in particular conductive pads (according to the English expression of the field) intended to be electrically connected to interconnection elements (pins of a package, matrix of balls of an interconnection substrate "interposer") constituting a first level of interconnection between the chip and the rest of the system in which it is intended to be integrated.To facilitate this first level of assembly, it is usual to place solder bumps on the conductive pads, these bumps being able to have a lateral dimension in the plane of the wafer (a width) and a perpendicular dimension (a height) to this plane typically between 15 microns or 20 microns to 250 microns.

[0071] The wafer, once provided with these elements, therefore has a surface topology (i.e. a 3D elevation profile) conforming to the arrangement of the solder bumps on the main face of the wafer, on the conductive pads of the chips of this wafer.

[0072] The placement of the solder bumps on the chips of the wafer is an operation that must be controlled, because a bad arrangement of these bumps (an incorrect positioning, the absence or an insufficient or excessive height of a bump or a plurality of bumps) can lead to forming a non-functional component. It is noted that this inspection can implement a 2-dimensional survey, in the main plane of the wafer, of the surface condition S of this wafer, for example to identify a bad positioning of a bump, or the presence of a parasitic deposit or particles on the surface of the wafer. The inspection can also collect height information, for example during a 3-dimensional survey, to identify that the respective maximum elevations of the bumps, or at least the respective maximum elevations of the bumps of the same chip, reside in the same plane or present a controlled variation in elevation around the same plane.

[0073] This mode of implementation therefore proposes to use the inspection device 1 which has just been described in order to inspect the surface S of the wafer and to ensure that its topology is indeed in conformity with a determined elevation profile.

[0074] Thus, and according to a first step of setting up the inspection method which is the subject of this mode of implementation, the wafer is placed on the mobile support 4 of the inspection device 1. The surface S of this wafer, that having the topology to be inspected, is of course oriented so as to be able to be exposed to the inspection light beam. The mobile support 4 is also adjusted in position according to the direction of the axis AO of the chromatic system 3, so as to position the surface S of the object at least partly in the depth of field.

[0075] The inspection device 1 is also operated via, for example, the control unit 7, during a second step of activating the polychromatic light source 20, in order to project the inspection beam onto the surface S of the wafer and thus form the reflected beam.

[0076] During a third measurement step of the method, the control unit 7 moves the surface S of the wafer relative to the inspection beam to travel a measurement trajectory, for example by moving the mobile support 4.

[0077] The control unit 7 synchronizes at the same time the image sensor 5 and in particular the accumulation and the displacement of the charges, from line to line, of the matrix of photodetectors, these charges being those generated by the projection of the light beam reflected on some of the photodetectors through the masks 6a, 6b.

[0078] The image sensor 5 repeatedly prepares the charge lines extracted from the matrix of photodetectors at the end of the columns, and these measurements are made available to the control unit 7 which acquires them and prepares an intensity image during a fourth step called “acquisition”.

[0079] Thanks to the inspection device according to the invention, the speed of movement of the beam with respect to the surface S can be rapid and reach for example a value of the order of 100 mm / second or more, while collecting sufficient charges to form a representative image of the state of the surface S.

[0080] At the end of this recording step, we therefore have an image of at least part of the surface S of the wafer, this part corresponding to the portion of the surface S of the wafer illuminated by the inspection beam after the latter has traveled along the measurement path. This image is composed of pixels respectively associated with measurement points of the surface S of the wafer. The value of each pixel reflects the intensity of the radiation reflected by the associated measurement point of the surface S, that is to say the degree of presence of this measurement point in the very extended depth of field of the chromatic system 3 (which can for example extend over a distance of 100 to 200 microns, as already stated).By assuming that the average elevation plane of the surface S is arranged, during the installation step, at one of the focal planes furthest from the depth of field, the image makes it possible to identify the arrangement on the surface of the wafer of the protruding patterns, and in particular the solder bumps. The analysis of the image makes it possible in particular to determine whether this arrangement of protruding elements is in accordance with the expected arrangement, or whether elements are missing (unformed or poorly formed solder bump) or whether elements are in excess (particles, parasitic deposit, etc.).

[0081] In the exemplary application, the solder bumps forming the protruding elements that are to be located are arranged on the surface of the wafer in an orderly manner, for example in lines. In such a configuration, it is advantageous to prepare the measurement path so that it is orthogonal or parallel to these lines.

[0082] It is also possible to provide for illuminating a portion of the wafer a plurality of times using the inspection beam, for example during a plurality of passes forming a measurement trajectory or a plurality of such trajectories, each pass being made with a different altitude. In this way, it is possible to compensate for a possible variation in spectral reflectivity of the surface S at a measurement point, by placing this point at a focal plane different from the depth of field of the chromatic system 3, and therefore by illuminating it with light radiation having different wavelengths.

[0083] For example, the different passages can be offset by a distance of a few microns to a few tens of microns.

[0084] As indicated in a previous passage of this description, the polychromatic source 20 can be chosen very freely, in particular to emit white light or to present a spectrum in lines.

[0085] According to a very advantageous variant of the method, the source 20 is chosen to emit according to a line spectrum, and the number of lines of this spectrum is limited for example to 2, 3, 5 or 10 wavelengths. The depth of field of the chromatic system 3 is then defined by a limited number (2, 3, 5 or 10) of focusing planes. When a protruding element of the wafer (a solder bump, a particle, a parasitic deposit) intercepts one of these planes, it is identified on the measurement image.

[0086] As an illustration of this variant, the polychromatic source 20 provides radiation composed of two wavelengths. The first wavelength is associated with a basic focusing plane in the depth of field, and this basic plane is intended to be aligned with the main plane of the wafer (i.e. the plane on which the protruding elements reside). This alignment can be carried out during the first step of setting up the method.

[0087] The second wavelength is associated with a second focusing plane distant from the first plane by the expected height of the protruding elements that one seeks to locate, or by a little less than this expected distance. By carrying out the inspection method using equipment conforming to the description and provided with such a source, an image is produced of at least part of the surface S of the wafer, this image having intensity peaks which correspond to protruding elements having at least a height corresponding to the expected height. It is possible to identify on this image an absence of a protruding element or a protruding element having an insufficient height (for example a solder bump) if the area of ​​the image where this element should be located does not have any light intensity.

[0088] The image produced using the method just described is essentially a 2D image of the surface of the wafer which contains little or no information on the elevation profile of the surface S inspected. This image is nevertheless very useful for identifying certain types of defects very quickly and over a large surface area of ​​the wafer.

[0089] To further improve the inspection capability, a method according to the invention may also take advantage of the color or spectral band distribution information that may be provided by the time-delayed integration image sensor 5 of an inspection device 1 according to the present description. As mentioned in a previous passage, determining the wavelength of the radiation detected by the photodetectors of the image sensor 5 makes it possible to determine the depth of a measurement point in the depth of field of the color system 3, i.e. the height of this measurement point relative to the main plane of the wafer.

[0090] The color information (RGB or more spectrally detailed) returned by the image sensor 5, in addition to the intensity information of the detected radiation, can be converted, for example by the control device 7, into height information. The image provided in this case, which can also materialize in the form of a point cloud, is a 3D image of the surface of the wafer and represents the topology of this plate, i.e. its elevation profile over its entire extent.

[0091] This image can be used very directly to determine the maximum height of the solder bumps, and to check whether this height is correct.

[0092] The color information provided by the image sensor 5 can be used in all the variations of the inspection method which have just been described, in particular that according to which the polychromatic source 20 is chosen to emit according to a spectrum having a limited number of lines.

[0093] Of course, the invention is not limited to the embodiments described and variant embodiments can be made without departing from the scope of the invention as defined by the claims.

[0094] Thus, the two opaque lighting and detection masks 6a, 6b can be replaced by a single mask, arranged in this case under the separating element 3b. The mask can for example be fixed directly under a separating cube or a reflective plate, for example by depositing an opaque material on this optical part. In this alternative configuration, the openings of the single mask perform both the function of the lighting and chromatic filtering openings. The openings of this mask are then optically conjugated to the surface S of the object by the chromatic system 3, and optically conjugated to the image sensor 5.

Claims

Claims

1. A method of inspecting a surface (S) of an object, the method comprising: - a first positioning step during which the object is placed on a mobile support (4) of an inspection device (1) so as to position the surface (S) in a depth of field of a chromatic system (3) of this device (1), the chromatic system (3) spatially spreading the focusing of a polychromatic inspection beam according to focusing planes in said depth of field arranged along an optical axis (AO), and intercepting a beam reflected by the surface (S) to project it onto a matrix of photodetectors of a time-delayed integration image sensor (5) arranged in a detection plane (P), conjugate of the focusing planes, the matrix of photodetectors being arranged according to a plurality of rows and columns; - a second step of activating at least one polychromatic light source (20) to project the inspection beam onto the surface (S) and thus form the reflected beam, at least one confocal mask (6a, 6b) having a plurality of chromatic filtering apertures intercepting the inspection beam and the reflected beam, the chromatic filtering apertures of the confocal mask (6a, 6b) making it possible to illuminate a portion of the photodetectors; the chromatic filtering apertures being distributed in a complementary manner over at least two lines of the confocal mask (6a, 6b) to illuminate a plurality of photodetectors corresponding to a plurality of measurement points arranged along at least two lines arranged along a direction perpendicular to an inspection direction (I); - a third survey step during which the mobile support (4) is controlled to move the surface (S) relative to the chromatic system (3) at least along the inspection direction (I) perpendicular to the optical axis (AO) and travel a measurement trajectory, and during which the image sensor (5) is controlled to synchronize it with the movement of the surface (S), the charges generated by the photodetectors being moved from one row to another of the matrix of photodetectors, along a column direction synchronized with the movement along the inspection direction (I); - a fourth stage of acquisition of data established by the sensor image (5) and preparation of an image formed of pixels respectively associated with measurement points of the surface (S) of the object, each pixel reflecting the intensity of the radiation reflected by the measurement point.

2. An inspection method according to the preceding claim wherein the object is a chip wafer, the wafer being provided with a plurality of solder bumps.

3. An inspection method according to the preceding claim wherein the weld bumps are arranged in a line, and the trajectory is at least partly parallel or perpendicular to the weld bump lines.

4. Inspection method according to one of the preceding claims in which the trajectory is formed of a plurality of passages having different altitudes.

5. Inspection method according to one of the preceding claims in which the third and / or the fourth step are implemented by a control unit (7) of an inspection device (1).

6. Inspection method according to one of the preceding claims comprising a step of analyzing the image to identify the arrangement of projecting elements of the surface (S).

7. Inspection method according to the preceding claim in which the analysis step leads to determining whether the arrangement of protruding elements conforms to an expected arrangement.

8. Inspection method according to one of the preceding claims in which the image sensor is a color or multi-spectral image sensor providing color or spectral information, and the method comprises a step of converting the provided color or spectral information into distance information to form a 3D image of the surface (S) of the object.

9. Inspection method according to one of the preceding claims in which the chromatic filtering openings are arranged on the confocal mask (6a, 6b) to illuminate a plurality of photodetectors arranged on a column of the matrix corresponding to a plurality of measurement points arranged according to the inspection direction (I).

10. Inspection method according to one of the preceding claims in which the confocal mask (6a, 6b) comprises a plurality of groups of at least two lines comprising chromatic filtering openings distributed in a complementary manner on said at least two lines.

11. Inspection method according to one of the preceding claims comprising a confocal illumination mask (6a) arranged in the inspection beam and a confocal detection mask (6b) arranged in the reflected beam, the confocal illumination mask (6a) and the confocal detection mask (6b) being respectively placed in a conjugate plane of the focusing planes with respect to the chromatic system (3).

12. Inspection method according to one of claims 1 to 10 comprising a single confocal mask, arranged in a jointly conjugate plane of the detection plane (P) and the focusing planes with respect to the chromatic system (3).

13. Inspection method according to one of the preceding claims in which the polychromatic light source (20) comprises a halogen lamp or a strip of light-emitting diodes, associated with a diffusion device (21) to homogenize the intensity of the inspection beam.

14. Inspection method according to one of the preceding claims in which the polychromatic light source (20) has a line spectrum, having a determined number of emission wavelengths or wavelength bands.

15. Inspection method according to one of the preceding claims in which the chromatic system (3) comprises a chromatic objective (3a).

16. Inspection method according to one of the preceding claims in which the chromatic system (3) comprises a separating element (3b).

17. Inspection method according to one of the preceding claims in which the photodetectors are provided with a color filter or a spectral filter.

18. Inspection method according to the preceding claim, in which the chromatic filtering apertures are arranged to each illuminate a plurality of photodetectors provided with filters of different colors or spectral bands.

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