Patent Description:
Information on the topography of a surface of an object is required in various areas of manufacturing. An area where the need for such information is particularly prominent is semiconductor manufacturing, where the semiconductor devices need to be inspected to ensure proper function. Such inspection includes specific structures making up the devices on a wafer, but also entities like solder bumps, which are required for holding components of a device together. For example, a die cut from a wafer may first be contacted to pins of a chip with an array of solder bumps. The chip can then be contacted to external circuitry by solder balls. For quality assurance, the heights of the solder bumps and solder balls with respect to the substrate have to be inspected before completion of the soldering.

Several methods for 3D topography measurements are well known in the art. Among these methods are white light interferometry, confocal microscopy, methods based on structured illumination, and laser triangulation with stereo vision. All these methods have their specific advantages and disadvantages.

White light interferometry is capable of providing height information of very high precision. The surface is moved in the interferometer by steps smaller than one wavelength; therefore, when inspecting semiconductor devices, a large number of frames of the surface needs to be taken and processed, as the steps have to extend over a range comparable with the height variation occurring on the surface.

Both confocal microscopy and methods based on structured illumination require rather standard microscope optics. Both approaches are better suited for inspecting surface topography at the scale of typical semiconductor devices. While confocal microscopy generally provides better height resolution than methods based on structured illumination, it also requires a more complicated and expensive optical setup.

The basic concept of methods based on structured illumination is to project a pattern, for example a grating, onto the surface of the object. There are two general approaches.

For an imaging system with low numerical aperture (NA), for example below <NUM>, for which a longer working distance and a greater depth of focus are possible, the pattern can be projected onto the surface at an angle with respect to the imaging optical axis. Such an arrangement is similar to laser triangulation, as the fringe phase shift instead of the position shift of a line illumination is used to extract surface height. This approach is also known as phase shift fringe projection method.

In case of an imaging system with higher NA, above <NUM>, neither oblique projection nor oblique imaging is easily implemented, as both depth of focus and working distance are limited. Here, instead, the pattern, for example a grating, is projected onto the surface through the imaging optics, and the optical axis of the imaging optics is normal to the surface of the object, more precisely to the plane defined by the general macroscopic extension of the surface. Due to this arrangement, height information cannot be extracted from fringe phase shift. Instead, height information can be obtained by moving the object in a direction parallel to the optical axis, and finding the position shift along this direction at which the contrast of the projected pattern is maximum.

There is a similarity between this setup and a confocal microscope, but the optics is simpler, not requiring relay optics. However, a higher data rate is required, as extracting the contrast of the pattern image requires three or more frames for each height position.

One example of such an approach, of structured illumination normal to the surface, can be found in <CIT>, issued on application <CIT>. A pattern is generated by a spatial light modulator (SLM) and projected onto the surface of an object along an optical axis of an imaging objective. The object is moved relative to the objective along the optical axis, while the SLM modulates the projected pattern and a plurality of images are recorded. Maximum contrast of the projected pattern at a particular position on the surface yields height information for the respective position.

Which of the methods for 3D topography measurement mentioned above is best depends on the requirements of the specific measurement application. For semiconductor device inspection, some key requirements are: a resolution in the plane defined by the macroscopic extension of the surface of a few µm, a repeatability of positioning the object along a direction normal to this plane of less than <NUM>, a total range of movement along this normal direction of a few hundred µm. In view of this, methods based on structured illumination appear to be the most suitable for semiconductor device inspection by 3D topography measurements. The configurations of pertinent systems can cover a wide range both of resolution in the plane of the surface and of repeatability normal to the plane, and the methods achieve a large range of relative movement along the normal direction. The optics is comparatively simple and low cost, the setup of illumination and imaging along the normal direction is suitable for a wide variety of surface types, including both surfaces with predominantly specular reflection and surfaces with predominantly diffuse reflection. In particular with respect to the inspection of solder bumps, a larger NA yields a larger number of usable pixels at the spherical bump top of smaller bumps.

While the basic concept of structured illumination outlined above and exemplified in the cited <CIT> achieves the required precision and accuracy, an unresolved problem is how to achieve these required characteristics while at the same time meeting ever increasing throughput requirements at preferably low cost, moreover in a manner that is scalable. For example, the spatial light modulator of the cited patent <CIT> used for generating the patterned illumination is expensive, yet does not have the resolution and pixel counts for covering a large field of view, which, however, would be essential for higher throughput.

<CIT> discloses a device and method for three dimensional optical mapping of a sample.

It is an object of the invention to provide a method for three-dimensional topography measurement of a surface of an object, which is easily implemented, provides sufficient in-plane resolution and repeatability along the normal direction, and is scalable.

It is a further object of the invention to provide a system for three-dimensional topography measurement of a surface of an object, which is of simple configuration, provides sufficient in-plane resolution and repeatability along the normal direction, and is modular and compact so as to be scalable.

The object regarding the method is achieved by a method according to claim <NUM>.

The object regarding the system is achieved by a system according to claim <NUM>.

The nature and mode of operation of the present invention will now be more fully described in the following detailed description of the invention taken with the accompanying schematic drawing figures.

Same reference numerals refer to same elements or elements of similar function throughout the various figures. Furthermore, only reference numerals necessary for the description of the respective figure are shown in the figures. The shown embodiments represent only examples of how the invention can be carried out. This should not be regarded as limiting the invention.

<FIG> shows an embodiment of a system <NUM> for 3D topography measurements of a surface <NUM> of an object <NUM>. The system <NUM> has a source of patterned illumination <NUM>; in the embodiment shown, the source of patterned illumination <NUM> has a light source <NUM>, for example one or plural LEDs, condenser optics <NUM>, and a pattern mask <NUM>. Patterned illumination of the surface <NUM> of the object <NUM> is generated by projecting the pattern mask <NUM> onto the surface <NUM>. More precisely, in the embodiment shown, light from light source <NUM>, after passing condenser <NUM> and pattern mask <NUM>, reaches beam splitter <NUM>, which directs at least a portion of the light towards objective <NUM>, through which the light reaches surface <NUM> of object <NUM>. Light from the surface <NUM> then passes through the objective <NUM> and reaches beam splitter <NUM>, which directs a portion of the light from the surface <NUM> to a detector <NUM>, which, as is shown here, may be part of a camera <NUM>. The objective <NUM> defines an optical axis <NUM>, and a focal plane <NUM>; the optical axis <NUM> is perpendicular to the focal plane <NUM>. The projected pattern mask <NUM> is at best focus in the focal plane <NUM>.

Via the detector <NUM> a plurality of images of the surface <NUM> are recorded, while a relative movement is performed between the object <NUM> and the objective <NUM>. A direction <NUM> of the relative movement between the object <NUM> and the objective <NUM> includes an oblique angle <NUM> with the optical axis <NUM>. During the relative movement, the surface <NUM> of the object <NUM> passes through the focal plane <NUM> of the objective <NUM>. In this macroscopic view of the system <NUM>, the focal plane <NUM> is shown coincident with the surface <NUM> of the object <NUM>. Parts of the surface <NUM> which lie in the focal plane appear at best focus in the images recorded of the surface <NUM> via the detector <NUM>. Due to the oblique angle <NUM> between the direction <NUM> of relative movement and the optical axis <NUM>, the pattern of the patterned illumination moves relative to the surface <NUM> of the object <NUM>; in addition, the contrast of the pattern, as recorded in the images of the surface, changes, as the surface <NUM> passes through the focal plane <NUM> over the course of the relative movement along direction <NUM>. As a result, the light intensity recorded from a position on the surface <NUM> varies between the images of the plurality of images. From this variation of the light intensity, height information for the respective position on the surface <NUM> can be obtained. For the sake of completeness we mention that the relative movement between the object <NUM> and the objective <NUM> may for example be achieved by moving the object <NUM> or by moving the system <NUM>, or by moving both the object <NUM> and the system <NUM>.

<FIG> is a schematic enlarged view of a part of the surface <NUM> of the object <NUM>, showing that the surface <NUM> generally is not flat, but has structures, like for example the elevation <NUM>. The 3D topography measurements the invention is concerned with aim at obtaining information on a height <NUM> of these structures, here explicitly shown for elevation <NUM>. The height <NUM> of the elevation <NUM> is understood as the extension of the elevation <NUM> relative to a reference plane <NUM>, along a direction perpendicular to the reference plane <NUM>. Also shown are the optical axis <NUM> and the pertinent focal plane <NUM> of the objective <NUM> (see <FIG>). If the object <NUM> is correctly aligned in the system <NUM>, the focal plane <NUM> is parallel to the reference plane <NUM>, and therefore the optical axis <NUM> is perpendicular on both the focal plane <NUM> and the reference plane <NUM>.

<FIG> is a top view of the surface <NUM> of the object <NUM>, showing a projected pattern <NUM>, originating from the source of patterned illumination <NUM> (see <FIG>). In the example shown, and with reference to the preceding figures, the object <NUM> is moved along direction <NUM> relative to objective <NUM>, so that this relative movement has a component <NUM> in the reference plane <NUM>. As a result, the pattern <NUM> moves in direction <NUM>, opposite to component <NUM>, with respect to the surface <NUM> of the object <NUM>. This implies that during the relative movement the light intensity incident on a given position on the surface <NUM> will vary, and as a result, the light intensity recorded from this position will vary between the images recorded of the surface <NUM> by camera <NUM>.

<FIG> shows an object <NUM>, the reference plane <NUM>, and an optical axis <NUM> of an objective <NUM> (see <FIG>); the optical axis <NUM> is perpendicular to the reference plane <NUM>. The object <NUM> has portions <NUM> and <NUM>, the height values of which, relative to the reference plane <NUM>, differ by an amount <NUM>, so the object <NUM> has a step <NUM>. In <FIG> the corresponding intensity signal, as obtained by the method according to the invention, is shown in a diagram. In the diagram, the abscissa <NUM> corresponds to a position of the object <NUM> along the optical axis <NUM>, and the ordinate <NUM> corresponds to light intensity recorded from a position on the object <NUM> during relative movement, here more precisely from the position of the step. The light intensity shows two portions of pronounced modulation, <NUM> and <NUM>. Assuming that in the case shown increasing values along the abscissa <NUM> correspond to a movement of the object towards the objective, modulation portion <NUM> results from the passage of portion <NUM> (greater height, closer to the objective than portion <NUM>) through the focal plane of the objective, and modulation portion <NUM> results from the passage of portion <NUM> through the focal plane of the objective. The difference between the positions of the maxima of the modulation portions <NUM> and <NUM> on the abscissa <NUM> corresponds to the height difference <NUM> between portion <NUM> and <NUM> of the object <NUM>. The high frequency modulations in the light intensity, discernible in particular within the modulation portions <NUM> and <NUM>, result from the combined effect of the pattern and the relative movement between object and objective. For example, if the pattern is a line pattern, these high frequency modulations result as bright and dark lines of the pattern pass over the step <NUM> of object <NUM> of <FIG>. The amplitude of these high frequency modulations, on the other hand, is determined by the contrast of the line pattern on the surface of the object, here more precisely on the portions <NUM> and <NUM>, respectively, of object <NUM>. The contrast is highest, and thus the amplitude of the high frequency modulations is highest, if the portion <NUM> or <NUM>, respectively, is in the focal plane <NUM> of the objective <NUM> (see <FIG>).

<FIG> shows an optical configuration for an illumination branch as can be used for the method according to the invention. Beam splitter and camera, as in <FIG>, are not shown. The optical configuration shown in <FIG> is used to discuss measurement uncertainty and possible improvements in this field, as precise measurements of height are essential to the invention.

Shown are, as in <FIG>, light source <NUM>, condenser <NUM>, grating <NUM>, objective <NUM> with optical axis <NUM>, and surface <NUM> of object <NUM>. Objective <NUM> includes pupil <NUM>, defining an imaging numerical aperture (imaging NA) <NUM>. Also indicated is illumination NA <NUM>.

For the following discussion, we introduce Cartesian coordinates, coordinate z along the optical axis <NUM>, and coordinate x perpendicular thereto.

In any plane perpendicular to the optical axis <NUM>, the intensity I of an image of the grating projected onto the plane can be expressed as <MAT>.

Here C(z) specifies the amplitude of intensity modulation as a function of z, Λ is the grating pitch, i.e. the distance between two neighboring lines of the grating <NUM>, and Φ is a phase offset. In order to measure the contrast and to ultimately determine the maxima of modulation portions like <NUM> and <NUM> shown in <FIG>, the fringe pattern is shifted in x-direction, which in the method according to the invention is accomplished by the oblique angle of the relative movement between the object and the objective, see also arrow <NUM> in <FIG>. A number M of such fringe pattern shifts are made over the distance of one grating pitch, or put differently, M images are recorded while the pattern is shifted by one grating pitch due to the relative movement. The corresponding intensity values for example are <MAT> where m is counting the fringe pattern shifts, <NUM>≤m≤M. The minimum value of M is <NUM>, but preferably M is <NUM> or even higher. The fringe contrast can be evaluated from the "M-bucket" algorithm, described by the following calculation steps: <MAT> <MAT> <MAT> <MAT>.

If, for instance, a one-dimensional sinusoidal grating is used, the contrast of the projected image of the grating as a function of z changes approximately as <MAT> where NAi is the numerical aperture <NUM> of the illumination, NA is the imaging numerical aperture <NUM>, λ is the wavelength (or mean wavelength) of the light used for illumination, and C<NUM> is the maximum fringe contrast at best focus.

Error propagation theory yields the following expression for the variance of the fringe contrast <MAT> which can be shown to give <MAT>.

Here <σI> is the average noise of pixel intensity, and <σI>/I<NUM> is the inverse of detector dynamic range in the sensor noise limited case, and the inverse of the square root of the full well capacity of the sensor in the shot noise limited case.

The slope of focus response at <NUM>% of the peak can be used to estimate measurement repeatability, giving <MAT> where N is the number of z-steps in the depth of focus. The measurement repeatability can then be expressed as <MAT> with Nt =MN indicating the total number of measurements, resulting from M fringe shifts at each of N z-steps, where a z-step is the change of position along the optical axis <NUM> while the projected pattern moves by one grating pitch, due to the relative movement between object and objective.

The goal of developing this error propagation model is to show how optics parameters affect performance at a fundamental level, so it is derived under ideal conditions in which mechanical motion error and sensor noise are ignored. This model represents the best case scenario. The preceding equation for the measurement repeatability shows that the measurement repeatability can be improved by:.

Therefore smaller grating pitch and higher grating contrast are preferred. However, grating pitch and fringe contrast are generally two conflicting requirements because fringe contrast decreases with smaller grating pitch, as shown in <FIG> for the optical transfer function of an incoherent imaging system with a circular aperture. In <FIG> the grating pitch is shown as spatial frequency of the grating, normalized to the maximum spatial frequency used. High spatial frequency means many grating lines per unit length, and thus a small distance between neighboring lines of the grating, i.e. a small grating pitch.

For incoherent illumination, the fringe contrast as a function of grating pitch is given by: <MAT> <MAT> <MAT>.

The measurement repeatability error as a function of grating pitch is obtained by combining these equations and the preceding equation for σz; the result is plotted in <FIG>. The optimum grating pitch is a little above twice the cut-off pitch Λmin, for simplicity, it is written as: <MAT>.

Therefore, for full NA illumination and shot noise limited case, the measurement repeatability is given by: <MAT>.

And in case of shot noise limited case: <MAT>.

Here Ne indicates the full well capacity of the imaging sensor. This is the best scenario case, to show the basic limit of the measurement performance. Real measurement is often limited by mechanical noise, mainly from the z-positioning stability.

As can be seen from <FIG>, the projected grating contrast at the optimum grating pitch (at half of cut-off frequency) is about <NUM>%, given by the modulation transfer function (MTF) of an incoherent imaging system. The low contrast is a result of the unbalanced mixing of diffraction orders at the object plane on which the grating is projected. This is further illustrated in <FIG> and <FIG>.

<FIG> shows condenser <NUM>, grating <NUM>, objective <NUM> with optical axis <NUM> and pupil <NUM>, and object <NUM>. Also indicated are the depth of focus <NUM> and the grating pitch <NUM>, as appears projected on the object <NUM>. The indications 0th, +<NUM>, and -<NUM> refer to the <NUM>th diffraction order, as well as to the two first order diffractions. The grating here is assumed to have a pitch equal to the wavelength of the light used for illumination divided by the numerical aperture of the pupil <NUM>.

<FIG> shows that, for the setup of <FIG>, for any given point on the illumination pupil, only one of the two first order diffractions (i.e. either +<NUM> or -<NUM>) passes through the optics, while the other is diffracted to outside of the pupil. The image of the grating <NUM> on the surface of the object <NUM> thus is formed either from diffraction orders <NUM> and +<NUM>, or from diffraction orders <NUM> and -<NUM>, which regenerate the image of the grating by interference. As the intensities of light in one of the first orders is lower than the intensity of light in the <NUM>th order for a standard grating, the resulting image of the grating has a low contrast.

<FIG> shows how to improve contrast. Shown are condenser <NUM>, grating <NUM>, objective <NUM> with pupil <NUM>, and object <NUM>. Also indicated are the depth of focus <NUM>, and the grating pitch <NUM>, as appears on the object <NUM>. The grating <NUM> here is assumed to have a pitch equal to the wavelength of the light used for illumination divided by the numerical aperture of the pupil <NUM>. The grating <NUM> here is such that it produces diffraction order <NUM> and only one first order diffraction, here -<NUM>, wherein the <NUM>th diffraction order and the single first order diffraction have equal intensity. This can for example be achieved with a blazed grating.

<FIG> shows that with the setup of <FIG>, the image of the grating is formed by interference of the <NUM>th diffraction order and, in the case shown, the diffraction order -<NUM>. As these two diffraction orders have equal intensity in the setup of <FIG>, the resulting image of the grating has an improved contrast in comparison with the situation shown in <FIG>. Contrast can in fact be improved to <NUM>%, leading to a corresponding improvement of measurement precision by more than a factor of <NUM>. Several variations of the setup of <FIG> are possible, for example an off-axis aperture.

Note that the improved contrast is not obtained at the expense of extended depth of focus. As shown in <FIG> and <FIG>, the geometric depth of focus, defined as distance from position of best focus at which the grating contrast has degraded to half maximum, both in the case of incoherent illumination, as in <FIG>, and for partially coherent off-axis illumination, shown in <FIG>, is roughly Λ/NA. For example, for nearly coherent illumination, where the numerical aperture for illumination, NAi, is much smaller than the imaging numerical aperture NA, the fringe pitch can be at minimum (corresponding to maximum spatial frequency) of λ/(2NA) and still have a fringe contrast of <NUM>%. A system where the contrast of the projected grating remains at <NUM>% through a practically infinitely large range of focus would have no height sensitivity on a specular reflective surface.

<FIG> shows an embodiment of a system <NUM> for 3D topography measurements of a surface <NUM> of an object <NUM>. The system <NUM> has a source of patterned illumination <NUM>; in the embodiment shown, the source of patterned illumination <NUM> has a light source <NUM>, for example one or plural LEDs, condenser optics <NUM>, and a pattern mask <NUM>. The system <NUM> also has a source of uniform illumination <NUM>; in the embodiment shown, the source of uniform illumination <NUM> has a light source <NUM>, for example one or plural LEDs, and condenser optics <NUM>. A means <NUM>, for example a beam splitter like a semi-transparent mirror, is provided for directing both light from the source of uniform illumination <NUM> and from the source of patterned illumination <NUM> to beam splitter <NUM>. Beam splitter <NUM> directs at least a portion of the light towards objective <NUM>, through which the light reaches surface <NUM> of object <NUM>. Light from the surface <NUM> then passes through the objective <NUM> and reaches beam splitter <NUM>, which directs a portion of the light from the surface <NUM> to a detector <NUM>, which, as is shown here, may be part of a camera <NUM>. The objective <NUM> defines an optical axis <NUM>, and a focal plane <NUM>; the optical axis <NUM> is perpendicular to the focal plane <NUM>. On the surface <NUM> of the object <NUM> a structure is shown, which in particular can be a specular structure, and here specifically is a solder bump <NUM>.

By operating the light sources <NUM> and <NUM> alternatingly, an alternating illumination of the surface <NUM> of the object <NUM> is provided. If the light source <NUM> is operated, i.e. caused to emit light, the illumination of the surface <NUM> of the object <NUM> is uniform. If the light source <NUM> is operated, i.e. caused to emit light, the illumination of the surface <NUM> of the object <NUM> is patterned.

Via the detector <NUM> a plurality of images of the surface <NUM> are recorded, while a relative movement is performed between the object <NUM> and the objective <NUM>. Some of the images of the plurality of images are recorded while the surface <NUM> is subject to uniform illumination, and some of the images of the plurality of images are recorded while the surface <NUM> is subject to patterned illumination. A direction <NUM> of the relative movement between the object <NUM> and the objective <NUM> in this embodiment is parallel to the optical axis <NUM>. During the relative movement, the surface <NUM> of the object <NUM> passes through the focal plane <NUM> of the objective <NUM>. In this macroscopic view of the system <NUM>, the focal plane <NUM> is shown coincident with the surface <NUM> of the object <NUM>.

As in the embodiment shown the direction of relative movement <NUM> is parallel to the optical axis <NUM> of the objective <NUM>, in contrast to the embodiment of <FIG> there is no shift of the projected pattern relative to the surface <NUM> of the object <NUM>. The embodiment of <FIG> is particularly aimed at inspecting surfaces with solder bumps. Solder bumps are typically laid out in arrays on the surface <NUM>, only one representative solder bump <NUM> is shown in <FIG>. In areas between solder bumps, where the distance between solder bumps is larger than the pitch of the pattern, e.g. grating, projected onto the surface, the height of the surface between the solder bumps can be measured from the contrast of the projected pattern without requiring a shift of the pattern relative to the surface. This implies that there is no need to record plural images for each relative position between object <NUM> and objective <NUM> along the optical axis <NUM>, as is necessary in prior art.

In this embodiment, the surface height between the solder bumps <NUM> is determined from images recorded under patterned illumination, while the height of the solder bumps <NUM> is determined from images recorded under uniform illumination.

We remark that, while in the embodiment shown in <FIG>, the source of patterned illumination <NUM> and the source of uniform illumination <NUM> have a light source each, this is not a limitation of the invention. Embodiments are conceivable in which the source for patterned illumination <NUM> and the source for uniform illumination <NUM> use a common light source. In such a case suitable means are provided for achieving the alternating illumination of the surface of the object by patterned and uniform illumination. Such means may for example be filters of switchable transmission, so that a path of light from the light source to the further elements of the source for patterned illumination and the source for uniform illumination, respectively, can be alternatingly blocked. The intensity of light from the respective sources of illumination may also be controlled by controlling the transmission of the respective filters. Alternatively, the means may also be such that they collect light from the light source and direct it alternatingly to the further elements of the source for patterned illumination and the source for uniform illumination, respectively.

<FIG> illustrates an optical situation when imaging a solder bump <NUM>, here of radius r. It turns out that, due to the surface curvature of the reflective solder bump <NUM>, only a small portion of the bump top can be imaged. The size of the bump top visible to the detector depends on both illumination numerical aperture and imaging numerical aperture. At full numerical aperture (NA) illumination, the full-width-half-maximum radius of the bump top visible to the detector is given by D=rNA. The optical NA needs to be large enough to provide enough optical resolution so that individual bumps in an array layout can be measured accurately. The bump layout is typically <NUM>:<NUM> ratio of bump spacing to bump diameter, therefore the imaging point spread function (PSF) needs to be on the order of the bump radius to avoid optical cross-talk between adjacent bumps. The minimum NA is therefore: <MAT>.

And the corresponding minimum diameter of a visible bump top then is <MAT>.

For device topography inspection, the typical NA is around NA=<NUM>-<NUM> in order to have a large field size to image the whole device and also to achieve high throughput, so the visible bump top is smaller than the optical PSF, therefore can be treated as a point object of the imaging system. In this case, either the peak pixel intensity or the size of the image of the bump top itself can be used for height measurement, since it follows closely how the imaging point spread function changes with focus.

<FIG> shows that, while a point P of the surface of the bump <NUM> may still be subject to illumination through pupil <NUM>, light reflected from this point P does not pass through the pupil <NUM>, and thus does not reach the detector <NUM> (see <FIG>). The point P of the surface of the solder bump <NUM> therefore is not visible in an image recorded by the detector <NUM>. It should be appreciated from <FIG> that this failure of the reflected light to pass through the pupil <NUM> is mainly due to the specular nature of the reflection combined with the curvature of the surface of the bump <NUM>.

<FIG> shows an operation sequence of the system shown in <FIG>, which illustrates the alternating illumination generated by the source of patterned illumination <NUM> and the source of uniform illumination <NUM> in <FIG>. The abscissa of the diagram shows a position z, which is the position of the object <NUM> along the optical axis <NUM> of the objective <NUM> (see <FIG>) during movement along direction <NUM>. The ordinate shows the intensity of the light emitted by the light sources <NUM> and <NUM>, respectively. A square <NUM> with a checkerboard pattern symbolizes the operation of the source for patterned illumination <NUM> (without the pattern being limited to checkerboard), and an empty square <NUM> symbolizes the operation of the source for uniform illumination <NUM>. Arrows pointing from the squares to bars <NUM>, <NUM> in the diagram indicate the stages of movement along the optical axis during which the respective source of illumination is active. So the source of patterned illumination <NUM> is active, i.e. provides illumination, for stages of the movement along the optical axis <NUM> where bars <NUM> are shown in the diagram, and the source of uniform illumination <NUM> is active, i.e. provides illumination, for stages of the movement along the optical axis <NUM> where bars <NUM> are shown in the diagram.

The bars <NUM> indicate a higher intensity of the light source <NUM> in the source of patterned illumination <NUM> than the bars <NUM>, which give the intensity of the light source <NUM> in the source of uniform illumination <NUM>. This is to show that the intensities of the light sources can be adapted to the properties of the portions of the surface <NUM> on which measurements are respectively performed. For measurements on the specular solder bumps, a lower intensity is normally adequate than for measurements on the surface between the solder bumps.

<FIG> shows, for the purpose of illustration only, a combined image <NUM> of solder bumps under uniform illumination and the surface <NUM> between the solder bumps under patterned illumination. Two diagrams are also shown. Diagram <NUM> gives, as a function of z-position, i.e. position along direction <NUM> (see <FIG>) parallel to optical axis <NUM>, the intensity recorded from a solder bump <NUM>. Diagram <NUM> gives, as a function of z-position, the contrast measured from the surface <NUM> between the solder bumps <NUM>. The intensity shown in diagram <NUM> has a maximum at a z-position <NUM>, the contrast shown in diagram <NUM> has a maximum at a z-position <NUM>. These z-positions <NUM>, <NUM>, where the respective maximum occurs, are the z-positions at which the top of the solder bump <NUM> (maximum <NUM>) and the surface <NUM> (maximum <NUM>), respectively, pass through the focal plane <NUM> (see <FIG>). The difference <NUM> between these z-positions <NUM> and <NUM> therefore is the height of the solder bump <NUM>.

As for the determination of the contrast values that enter diagram <NUM>, these can be calculated from a minimum of 2x2 pixels, if the projected pattern is a checkerboard pattern matched to pixel size of detector <NUM> (see <FIG>). A larger pixel area, i.e. an NxN pixel area with N><NUM>, can also be used. The choice will usually depend on distance between bumps <NUM> and spatial resolution requirements perpendicular to the optical axis <NUM>. Larger pixel areas lead to a higher precision of the calculated contrast, but, evidently, to lower spatial resolution perpendicular to the optical axis <NUM>.

<FIG> shows the pixel response (e.g. value of a pixel representing light intensity recorded by the corresponding pixel of the detector) in the method according to the invention for a surface of small curvature, like a solder bump. To the left of the figure, five images show light rays <NUM>, directed to a focus point <NUM> (only indicated in two of the images), impinging on the surface of a hemisphere <NUM> (solder bump), for different relative positions between the hemisphere <NUM> and the focus point <NUM>. The diagram to the right gives the pixel response on the ordinate <NUM>, while the abscissa <NUM> gives the relative position between the hemisphere <NUM> and the focus point <NUM>. The arrows indicate which parts of the pixel response in the diagram correspond to which of the five images on the left.

As can be seen, the pixel response has two maxima. The maximum having the smaller value of the abscissa <NUM> corresponds to the situation where the focus point <NUM> of the light rays <NUM> is at the top of the hemisphere <NUM>, as shown in the second image from below on the left. In a measurement, this situation occurs when the top of the solder ball is in the focal plane <NUM> of the objective <NUM> (see <FIG>). The second maximum occurs when the focus point <NUM> of the light rays <NUM> coincides with the center of the hemisphere <NUM>, as shown in the second image from above on the left; note that the light rays <NUM> do not actually penetrate into the hemisphere <NUM>, but get reflected on its surface. When performing a measurement, the direction of the relative movement between the object <NUM> and the objective <NUM> is known; therefore, it is unambiguously clear which of the two peaks corresponds to a top of a solder ball in the focal plane <NUM>. The other peak can be used to measure the curvature of the bump top surface, which in turn can be used for calibration purposes to improve measurement accuracy.

<FIG> shows several examples of pattern masks for generating the patterned illumination. These pattern masks can be used both in embodiments of the type shown in <FIG>, with patterned illumination only, and in embodiments of the type shown in <FIG>, with alternating patterned and uniform illumination. The invention is not limited to the types of pattern masks shown here. The specific examples shown are a sinusoidal grating (A), a checkerboard (B), a line grid or cross-line grating (C), and a pinhole array (D).

<FIG> shows an embodiment of a system <NUM> for 3D topography measurements of a surface <NUM> of an object <NUM>. The embodiment shown is very similar to the embodiment of the system <NUM> shown in <FIG>, where most of the elements that appear in <FIG> have already been discussed. In the system <NUM>, a pupil mask <NUM> is included in the source of uniform illumination <NUM>. The pupil mask <NUM> acts as an illumination aperture. An illumination aperture can improve image contrast and focus response of various feature shapes. Also shown in <FIG> are two, non-limiting, examples of possible shapes of pupil masks <NUM>. Pupil mask example <NUM> is a ring aperture, pupil mask example <NUM> is a circular aperture.

<FIG> shows an optics module <NUM> that is an embodiment of a system according to the invention and thus can be used for carrying out the invention. The configuration of the optics module <NUM> shown here is similar to the configuration of the system <NUM>, shown in <FIG>; an optics module based on the configuration of system <NUM> of <FIG>, or of system <NUM> of <FIG> may also be conceived of.

A source for patterned illumination <NUM> includes a light source <NUM>, a condenser <NUM>, and a pattern mask <NUM>. From the source for patterned illumination <NUM> the light reaches beam splitter <NUM>, which directs a portion of the light to objective <NUM>, from where it reaches object <NUM> and provides a patterned illumination of the surface <NUM> of the object <NUM>. The objective <NUM> includes a pupil <NUM>. Light from the surface <NUM> passes through objective <NUM> and beam splitter <NUM>, and then reaches detector <NUM> in camera <NUM>. Detector <NUM> is used to record a plurality of images of the surface <NUM> during a relative movement of the object <NUM> and the objective <NUM>, as has already been discussed above.

The module <NUM> is compact and simple, thus suitable for use in parallel inspection of plural objects. In order to provide a very specific, yet non-limiting example, the objective <NUM> may have a <NUM> field diameter, a NA of <NUM>, and may be corrected for typically <NUM> wavelength bandwidth of LED illuminations; this is preferred, as one or plural LEDs are typically used as light source <NUM>. The NA is large enough to achieve sub-µm measurement precision and the field size can cover most of the sizes of objects that are to be inspected. The beam splitter cube <NUM> in the imaging side splits the illumination path from the imaging path, and is an integrated part of the lens design. This is a much simpler and more compact design than the conventional imaging microscopes which have a separate objective lens and tube lens, and for which grating projection requires an additional tube lens since illumination and imaging path are split at the collimated space between objective and tube lens. Another advantage of this design is that pattern mask <NUM> and detector <NUM> are at exactly conjugate planes, therefore residual field distortion is cancelled and sampling aliasing of projected patterns is eliminated. The design is also telecentric on both object and image sides to minimize through focus signal distortion.

<FIG> shows a system <NUM> for parallel inspection of plural objects <NUM>. Objects <NUM> are placed on a conveyor <NUM> by pick-and-place device <NUM>. The conveyor <NUM> carries the objects <NUM> to and past an arrangement <NUM> of inspection modules <NUM>; in the specific example shown, the system <NUM> has three inspection modules <NUM>. Each object <NUM> is inspected by one inspection module <NUM>. Each inspection module <NUM> performs a method according to the invention on each object <NUM> it is used to inspect. It is also conceivable that the system <NUM> is configured such that the number of inspection modules <NUM> can be varied, i.e. that inspection modules <NUM> can be added to or removed from the system <NUM>, depending on the number of objects <NUM> to inspect and on throughput requirements.

Each inspection module <NUM> may for example be a module <NUM> as described in <FIG>, but may also be, for example, a system <NUM> as discussed in <FIG>, a system <NUM> as discussed in <FIG>, or a system <NUM> as discussed in <FIG>. The inspection module <NUM> may in general be any of the systems according to the invention discussed in the context of <FIG>, as well as any system configured to carry out a method according to the invention. The inspection module <NUM> may use the method according to the invention based on a patterned illumination and a relative movement between the object and the objective at an oblique angle between the direction of the relative movement and the optical axis of the objective, or the method according to the invention employing a patterned illumination and a uniform illumination alternatingly.

In the above description, numerous specific details are given to provide a thorough understanding of embodiments of the invention. However, the above description of illustrated embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise forms disclosed. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the invention. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention as defined in the appended claims.

These modifications can be made to the invention in light of the above detailed description.

Claim 1:
A method for optical three-dimensional topography measurement of a surface of an object (<NUM>), the method comprising the steps:
projecting patterned illumination through an objective (<NUM>) onto the surface of the object (<NUM>);
performing a relative movement between the object (<NUM>) and the objective (<NUM>), wherein a direction of the relative movement includes an oblique angle with an optical axis (<NUM>) of the objective, and wherein the surface passes through a focal plane of the objective during the relative movement, and wherein the surface of the object (<NUM>) defines a reference plane (<NUM>) which is parallel to the focal plane of the objective;
recording a plurality of images of the surface through the objective (<NUM>) during the relative movement;
deriving height information for a respective position on the surface of the object (<NUM>) from the variation of the intensity recorded from the respective position in the plurality of images, wherein each of the plurality of images is recorded as an array of pixels, each of the images being shifted such that a given position on the surface of the object corresponds to one and the same pixel in the array of pixels for all images of the plurality of images, wherein the values of a plurality of pixels in the array of pixels are combined by averaging.