Measuring device and method for measuring the thickness of a wafer

Abstract

The invention relates to a measuring device for measuring the thickness of a wafer (12) with two parallel interfaces (13, 36), having a measuring interferometer (42) which comprises a converging optics unit (46) and a light source (44) for generating a measurement light beam (56). The measuring interferometer (42) directs the measurement light beam (56) to the wafer (12) via the converging optics unit (46). The wavelength range of the measurement light beam (56) is chosen such that the wafer (12) is at least partially transmissive to the measurement light beam (56). The measuring device moreover has an evaluation unit (62) that calculates the thickness of the wafer (12) from interference signals provided by the measuring interferometer. According to the invention, the converging optics unit (46) is configured to vary the axial position of the focus of the measurement light beam (56) on the basis of a control signal. The control signal for the optical element (64) is defined such that a focus (58) of the measurement light beam (56) generated by the converging optics unit (46) is at least intermittently located on that interface (13) of the wafer (12) which is further away from the converging optics unit (46) in the light path of the measurement light beam (56).

Description

[0001] Measuring device and method for measuring the thickness of a wafer

[0002] BACKGROUND OF THE INVENTION

[0003] 1. Field of the invention

[0004] The invention relates to a measuring device and a method for measuring the thickness of a wafer, in particular for inline thickness measurement during a grinding, polishing or lapping process in which material is removed from one side of the wafer.

[0005] 2. Description of the state of the art

[0006] Wafers are usually circular or square discs with a thickness of around 1 mm, which serve as substrates for integrated circuits, micromechanical components, or photoelectric coatings. Semiconductor wafers are generally manufactured from monocrystalline or polycrystalline ingots, which are sawn into individual wafers perpendicular to their longitudinal axis. Besides silicon, semiconductors used include gallium arsenide, silicon carbide, and indium phosphide. Thin glass discs, which are used, for example, in the production of microlens arrays, are also referred to as wafers.

[0007] One side of the wafer, called the front side, carries the integrated circuits or other functional elements such as lenses. After the integrated circuits or other functional elements are built and before the wafer is diced and packaged, the back side of the wafer is often ground, polished, or lapped to remove unwanted material. A thin wafer has several advantages. For example, thermal resistance is reduced; it also reduces problems due to differences in thermal expansion coefficient. Material removal, also called back-grinding or back-thinning, often involves several steps using grinding wheels or polishing cloths with progressively finer grits.

[0008] Currently, wafers are ground back to thicknesses of approximately 300 pm. It is expected that in the near future, wafer thicknesses will be as low as 100 to 150 pm. Even now, these values ​​can only be achieved if the grinding process is continuously monitored. US 2002 / 0048901 A1 discloses optically measuring the wafer thickness while the backside of the wafer is being ground.

[0009] DE 10 2011 001 238 A1 discloses a monitoring device with an optical coherence tomograph that continuously measures the thickness during the grinding process. The coherence tomograph generates a measuring light beam of broadband coherent light, which is directed onto the back of the wafer. An interference signal resulting from the superposition of two components of the measuring light is detected in the coherence tomograph. A first component is formed by measuring light reflected from the back interface of the wafer facing the coherence tomograph. A second component is formed by measuring light that has passed through this back interface and been reflected from the opposite front interface of the wafer, on which the functional elements are located. The wavelengths of the measuring light lie in a range in which the absorption coefficient of the wafer is minimal.In this way, a sufficiently intense interference signal is obtained.

[0010] In practice, however, it has been shown that, especially with highly doped wafers, despite a suitable choice of wavelengths, the reflection from the interface further away from the measuring head of the coherence tomograph is sometimes so weak compared to the reflection from the closer interface that the interference contrast is not sufficient for a reliable measurement.

[0011] Currently, this problem is solved by using measuring heads with converging optics that have a large depth of field. If the focus is positioned approximately midway between the two wafer interfaces and the depth of field is sufficiently large, the two interfaces are always located within a range where the measurement light is fairly well focused, despite unavoidable variations in the positioning of the measuring head and wafer. This is advantageous because the better the light is focused at an interface, the stronger the reflection and, accordingly, the interference signal.

[0012] However, even when using collecting optics with a large depth of field, the interference signal occasionally does not stand out sufficiently from the noise. SUMMARY OF THE INVENTION

[0013] It is therefore an object of the invention to provide a measuring device with a measuring interferometer and a method for measuring the thickness of a wafer, in which the resulting interference signals are sufficiently strong even under unfavorable conditions.

[0014] With regard to the measuring device, this object is achieved by a measuring device for measuring the thickness of a wafer having two parallel interfaces, wherein the measuring device comprises a measuring interferometer having a collecting optics and a light source for generating a measuring light beam. The measuring interferometer is configured to direct the measuring light beam onto the wafer via the collecting optics. A wavelength range of the measuring light beam is selected such that the wafer is at least partially transparent to the measuring light beam. An evaluation unit is configured to calculate the thickness of the wafer from interference signals provided by the measuring interferometer. According to the invention, the collecting optics is configured to vary the axial position of the focus of the measuring light beam depending on a control signal.The control signal for the collecting optics can be set in such a way that a focus of the measuring light beam generated by the collecting optics lies at least temporarily on that boundary surface of the wafer which is further away from the collecting optics in the light path of the measuring light beam.

[0015] As already mentioned, the problems with interference signals that are too weak are primarily caused by the reflection at the far interface being too weak compared to the reflection at the near interface. Such a low interference contrast occurs particularly when the wafer, due to its doping, has a particularly high absorption for the measurement light penetrating the wafer. On the way to the far interface, such wafers absorb so much light that the reflection is too weak due to the large depth of field, despite the approximate focus.

[0016] The invention proposes improving the interference contrast by increasing the intensity of the reflection at the distant interface and simultaneously reducing the intensity at the near interface. This is achieved by positioning the focus of the measuring light beam generated by the collecting optics exactly on the distant interface of the wafer, at least temporarily. Due to the exact focusing on the distant interface, the intensity of the reflection there increases considerably. At the same time, the beam diameter widens at the near interface, whereby the intensity of the reflection created there decreases accordingly. This effect, i.e. the attenuation of the reflection at the near interface, can be significantly increased by reducing the depth of field during focusing. For this purpose, for example, a collecting optics with a higher numerical aperture can be used.

[0017] However, since the height of the wafer varies during grinding or when the polishing cloth is removed, it is not possible to permanently adjust the height of the collection optics so that the focus of the measuring light beam is permanently on the remote interface of the wafer. The measuring device must therefore be capable of varying the axial position of the focus so that it is at least temporarily on the remote interface. For this purpose, the collection optics preferably contains a liquid lens or another optical element whose collecting or dispersing effect can be varied by a control signal. This allows the axial position of the focus to be varied.

[0018] As an alternative to changing the collecting or dispersing effect of an optical element, the axial position of the focus can also be varied by mechanically shifting an optical element spatially, particularly in the axial direction, using a displacement device. The displacement device is controlled by the control signal, so that the axial position of the focus can be changed by the control signal.

[0019] Measuring devices with an adjustable focal length are known in the prior art, but are used for other purposes. For example, WO 2021 / 140396 A1 discloses a laser processing device with an optical-confocal distance measuring device comprising a focal-length-variable measuring light optics. In this known measuring device, the focal length of the measuring light optics is preferably cyclically adjusted to increase the measuring range of the distance measuring device.

[0020] Preferably, the evaluation unit is configured to calculate the thickness only from those interference signals that differ from a noise signal with an amplitude that is maximum or greater than a predetermined threshold. This ensures that no interference signals that are too weak and therefore unreliable contribute to the thickness measurement.

[0021] In one embodiment, the focus of the measuring light beam is permanently on the remote interface of the wafer. This can be achieved by an autofocus control system configured to set the control signal for the collection optics such that the interference signals provided by the measuring interferometer permanently stand out from a noise signal with an amplitude that is maximum or greater than a predetermined threshold. The autofocus control system thus continuously varies the axial position of the focus by appropriately controlling the collection optics and checks how this affects the amplitude of the interference signal. As soon as the amplitude reaches its maximum (or alternatively exceeds a predetermined threshold), a measurement is performed by calculating a thickness from the interference signal.

[0022] From a control engineering perspective, it is simpler if the focus of the measuring light beam is only temporarily on the remote interface of the wafer, because the focus sweeps over the remote interface intermittently. This can be achieved by a controller that is designed to set the control signal for the collection optics so that the focus of the measuring light beam varies periodically. If the variation in focus is sufficiently large, it will sweep over the remote interface of the wafer at regular intervals. The moment at which the focus is exactly on the remote interface can be identified by the interference signal reaching its maximum. This maximum interference signal is preferably evaluated by calculating the thickness from it.

[0023] The axial position of the focus can, for example, vary sinusoidally. This is advantageous because it makes it particularly easy to control a liquid lens. For typical measuring tasks, the period of the sine function can be between 100 and 200 ms. A period of 150 ms, for example, enables a measuring frequency of around 6 Hz. Six wafers can then be fed to the measuring device for measurement every second. The evaluation unit can be part of the measuring interferometer. The collecting optics is often located in a measuring head attached close to the wafer, which is connected to a more distant table-top device via an optical fiber and control lines. The table-top device then contains, for example, the light source, the detector of the measuring interferometer, a control system for the liquid lens and the evaluation unit.However, the evaluation unit can also be outsourced and implemented, in particular, as software in a PC, to which the interference signals are fed after conversion into electrical signals.

[0024] The invention therefore also relates to a measuring interferometer without an evaluation unit. The measuring interferometer has a collecting optics and a light source for generating a measuring light beam, wherein the measuring interferometer is configured to direct the measuring light beam via the collecting optics onto a wafer having two parallel interfaces. According to the invention, the collecting optics is configured to vary the axial position of the focus of the measuring light beam depending on a control signal. The control signal for the optical element can be set such that a focus of the measuring light beam generated by the collecting optics lies, at least temporarily, on the interface of the wafer that is further away from the collecting optics in the light path of the measuring light beam.

[0025] With regard to the method, the object posed at the outset is achieved by a method for measuring the thickness of a wafer having two parallel interfaces, the method comprising the following steps: a) a measuring interferometer generates a measuring light beam and directs the measuring light beam onto the wafer via a collecting optics, wherein a wavelength range of the measuring light beam is selected such that the wafer is at least partially transparent to the measuring light beam; b) the thickness of the wafer is calculated from interference signals provided by the measuring interferometer; c) the collecting optics are controlled such that a focus of the measuring light beam generated by the collecting optics lies, at least temporarily, on that interface of the wafer which is further away from the collecting optics in the light path of the measuring light beam. With regard to the method, the above explanations regarding advantageous embodiments apply accordingly.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In the following, exemplary embodiments of the invention are explained in more detail with reference to the drawings. In these drawings:

[0028] Figure 1 shows a grinding device for back-grinding processed wafers with a measuring device according to the invention for measuring the thickness of the wafer;

[0029] Figure 2 schematically illustrates the axial position of the focus of a measuring light beam generated by a conventional measuring device;

[0030] Figure 3a schematically illustrates the axial position of the focus of a measuring light beam generated by a measuring device according to the invention;

[0031] Figure 3b schematically illustrates how the axial position of the focus of the measuring light beam is adjusted according to the invention when the position of the wafer changes relative to the measuring device;

[0032] Figure 4 shows a section of the measuring device according to Figure 1, which periodically varies the position of the focus;

[0033] Figure 5 shows a graph in which the axial position of the focus is plotted as a function of time; and

[0034] Figure 6 shows a graph in which the intensity of the interference signal is plotted as a function of time.

[0035] DESCRIPTION OF PREFERRED EMBODIMENTS

[0036] 1. Wafer measurement

[0037] Figure 1 shows a grinding device, designated overall by 10, for back-grinding processed wafers 12, as well as a measuring device 14 according to the invention, which measures the thickness of the ground wafers 12. The grinding device 10 comprises a disk-shaped wafer holder 16, over the circumference of which a plurality of recesses 18 are distributed. A wafer 12 can be inserted into each recess 18. The wafers 12 can be, for example, thin disks of silicon that have already been processed and carry electronic circuits (not shown) on their front side 13 facing downwards in Figure 1. The wafer holder 16 can be rotated about a rotation axis 22 with the aid of a drive 20 in order to deliver the wafers 12 to a plurality of stations distributed over the circumference, of which only one measuring station 23 can be seen in the section in Figure 1.

[0038] Opposite the recesses 18 is a sanding cloth holder 24, to the underside of which a replaceable sanding cloth 26 is attached. During sanding, the sanding cloth holder 24 is rotated about a rotation axis 30 with the aid of an additional drive 28, whereby the sanding cloth 26 is moved over the wafer 12 and removes material from its rear side 36.

[0039] At the measuring station 23, there is a frame 32 that prevents a grinding fluid 34 from flowing out. The grinding fluid 34 is located between a protective glass 38 and the back side 36 of the wafer 12, which faces away from the recess 18 and runs parallel to the front side 13 of the wafer 12.

[0040] The measuring device 14 measures the thickness of the wafer 12 located in the measuring station 23 at a given time. The measuring device 14 comprises a measuring interferometer 42, which has a light source 44, a collecting optics 46, and a detector 48. The light source 44 generates broadband measuring light, which is fed to the collecting optics 46 via an optical fiber 50. The latter is thus separated from the other parts of the measuring device 14 and forms a compact measuring head 52 with its own housing 54, which can be arranged in a space-saving manner even in cramped conditions such as those prevailing at the measuring station 23. The remaining parts of the measuring device 14 can be housed together in a housing 55 of a table-top device, as shown schematically in Figure 1, or distributed among various individual devices. These remaining parts include, among others, a central control module 61 with an operating unit and an evaluation unit 62.The collecting optics 46 focuses the measuring light emerging from the optical fiber 50 and bundles it into a measuring light beam 56, which penetrates the protective glass 38 and the grinding fluid 34 and converges at a focus 58. A portion of the measuring light incident on the wafer 12 is reflected at the back 36 of the wafer 12. The remaining measuring light penetrates the wafer 12 and is partially absorbed there. The non-absorbed portion of the penetrated measuring light strikes the front 13 of the wafer 12 and is also partially reflected there. The two reflected measuring light components pass via the collecting optics 46 and the optical fiber 50 back into the housing 55 and are fed via a beam splitter 60 to the detector 48. This is designed as a spectrograph and detects an interference signal resulting from the superposition of the two reflected measuring light components.The intensity of the interference signal is detected as a function of wavelength and converted into electrical signals by detector 48. The evaluation unit 62 uses these electrical signals to calculate the thickness of wafer 12 at the location impinged by the measuring light beam 56. This measurement method is referred to as a 1D-se FD OCT method (one-dimensional spatially encoded Fourier Domain Optical Coherence Tomography). Alternatively, a 1D-th FD OCT method (one-dimensional time encoded Fourier Domain Optical Coherence Tomography) can be used, which, however, requires a tunable light source.

[0041] The calculated thickness values ​​are transmitted to a machine control system 63, which controls the drives 20, 28 for the wafer holder 16 and the grinding cloth holder 24. Once the wafer 12 has reached the desired thickness, the grinding process is aborted.

[0042] In this respect, both the grinding device 10 and the measuring device 14 are known, which is why further explanations are omitted.

[0043] The collecting optics 46 of the measuring device 14 preferably has a liquid lens 64 whose focal length can be changed by an electrical control signal. The liquid lens 64 is designed here as a collecting lens, but could also have a diverging effect, as long as the other lenses of the collecting optics 46 have a sufficiently strong collecting effect. The liquid lens 64 receives electrical control signals from a controller 65, which is also located in the housing 55 and can be part of the central control module 61. With the help of the liquid lens 64, the axial position of the focus 58 can be changed within certain limits. According to the invention, the focus 58 is positioned axially such that it is at least temporarily located on the front side 13 of the wafer 12.

[0044] The associated advantages are explained below with reference to Figures 2 and 3a and 3b.

[0045] Figure 2 shows the axial position of the focus 58, as it is fixed in conventional measuring devices. The focus 58 is located between the front side 13 and the back side 36 of the wafer 12. Since the measuring light beam 56 converges only weakly, the depth of field is large. The measuring light therefore covers a small area on both the front side 13 and the back side 36 of the wafer 12. If the position of the front side 13 and the back side 36 changes, as indicated by dashed lines in Figure 2, the covered area does not change significantly due to the large depth of field.

[0046] In the case of wafers 12 which, for example, due to doping, strongly absorb the measuring light beam 56 despite optimally selected wavelengths, the reflection on the rear side 36 can be so much stronger than on the more distant front side 13 that the interference contrast is too low and thus the interference signal after the Fourier transformation has an amplitude which does not stand out sufficiently from the noise.

[0047] Figure 3a illustrates the inventive axial position of the focus 58 precisely on the more distant front side 13 of the wafer 12. By focusing the measuring light beam 56 on the front side 13 of the wafer 12, which is further away from the collecting optics 46, a very small and intense light spot is created there, which leads to a correspondingly strong reflection. Since the collecting optics 46 has a larger numerical aperture than conventional collecting optics, the depth of field is smaller. Therefore, the measuring light is distributed over a larger area on the back side 36 of the wafer 12, which leads to a weaker reflection. These two effects significantly increase the interference contrast and thus the amplitude of the interference signal. If the axial position of the front side 13 and the back side 36 changes, as illustrated in Figure 3b for a wafer 12', the focus 58 of the measuring light beam 56 indicated by dashed lines would no longer be on the front side 13.Consequently, the focus of the collecting optics 46 must be adjusted using the liquid lens 64. The adjusted focus 58' of the measuring light beam 56' then lies exactly on the front side 13, so that the amplitude of the interference signal does not decrease, or does not decrease significantly.

[0048] Autofocus control ensures that the focus 58 always remains on the front side 13 of the wafer 12. The autofocus control evaluates the amplitude of the interference signal and varies the focal length of the liquid lens 64 until the amplitude is maximum.

[0049] A simpler solution consists in periodically varying the axial position of focus 58 according to a predetermined periodic function, as illustrated in Figure 4. There, several axially closely spaced foci 58' are indicated, which are generated sequentially by continuously tuning the liquid lens 64. At a certain point in time, one of the foci 58' is inevitably located on the front side 13 of the wafer 12, which is recognizable by a maximum amplitude of the interference signal. Only this signal is further evaluated to calculate the thickness of the wafer 12.

[0050] The graph shown in Figure 5 illustrates how the axial position Zf0us of the focus 58 varies over time t according to a sine function. The axial position of the front side 13 of the wafer 12 is indicated by a dashed line 68. At times t1, t2, and t3, the focus 58 lies on the front side 13, so that a measurement can be performed. Rectangles indicate dead times 70a, 70b during which no measurement is possible because a new wafer 12 is introduced into the measuring station 23. The period of the focus variation is dimensioned such that in the available time periods between dead times 70a, 70b, the focus 58 sweeps over the front side 13 at least once.

[0051] The graph shown in Figure 5 schematically illustrates the interference signal summed across all frequencies as a function of time t. At the times t1, t2, and t5, at which the focus 58 lies on the front side 13, the amplitude of the interference signal is maximum, so the measurement is performed at these times. 70 schematically indicates a noise level from which the interference signal must reliably stand out.

Claims

PATENT CLAIMS 1. A measuring device for measuring the thickness of a wafer (12) having two parallel interfaces (13, 36), comprising: a measuring interferometer (42) having a collecting optics (46) and a light source (44) for generating a measuring light beam (56), wherein the measuring interferometer (42) is configured to direct the measuring light beam (56) onto the wafer (12) via the collecting optics (46), and wherein a wavelength range of the measuring light beam (56) is selected such that the wafer (12) is at least partially transparent to the measuring light beam (56), and having an evaluation unit (62) configured to calculate the thickness of the wafer (12) from interference signals provided by the measuring interferometer, characterized in that the collecting optics (46) are configured to vary the axial position of the focus of the measuring light beam (56) depending on a control signal, and in that the control signal can be determined in such a waythat the focus (58) of the measuring light beam (56) generated by the collecting optics (46) lies at least temporarily on that boundary surface (13) of the wafer (12) which is further away from the collecting optics (46) in the light path of the measuring light beam (56).

2. Measuring device according to claim 1, characterized in that the collecting optics (46) contains an optical element (64) whose collecting or scattering effect can be changed by the control signal.

3. Measuring device according to claim 2, characterized in that the optical element is a liquid lens (64).

4. Measuring device according to one of the preceding claims, characterized in that the evaluation unit (62) is designed to calculate the thickness only from such interference signals which have an amplitude of one Noise signal that is maximum or greater than a specified threshold.

5. Measuring device according to one of the preceding claims, characterized by an autofocus control which is designed to set the control signal in such a way that the interference signals provided by the measuring interferometer (42) permanently stand out from a noise signal with an amplitude which is maximum or greater than a predetermined threshold value.

6. Measuring device according to one of claims 1 to 5, characterized by a controller (65) which is designed to set the control signal such that the focus (58) of the measuring light beam (56) varies periodically.

7. Measuring device according to claim 6, characterized in that the focus (58) varies according to a sine function.

8. Measuring device according to one of the preceding claims, characterized in that the light source (44) generates broadband measuring light and the measuring interferometer (42) comprises a spectrograph (48).

9. A measuring interferometer (42) comprising a collecting optics (46) and a light source (44) for generating a measuring light beam (56), wherein the measuring interferometer (42) is configured to direct the measuring light beam (56) via the collecting optics (46) onto a wafer (12) having two parallel boundary surfaces (13, 36), characterized in that the collecting optics (46) are configured to vary the axial position of the focus of the measuring light beam (56) as a function of a control signal, and in that the control signal can be set such that a focus (58) of the measuring light beam (56) generated by the collecting optics (46) lies at least temporarily on that boundary surface (13) of the wafer (12) which is further away from the collecting optics (46) in the light path of the measuring light beam (56).

10. Measuring interferometer according to claim 9, characterized in that the collecting optics (46) contains an optical element (64) whose collecting or scattering effect can be changed by the control signal.

11. A method for measuring the thickness of a wafer (12) having two parallel interfaces (13, 36), comprising the following steps: a) a measuring interferometer (42) generates a measuring light beam (56) and directs the measuring light beam (56) onto the wafer (12) via a collecting optics (42), wherein a wavelength range of the measuring light beam (56) is selected such that the wafer (12) is at least partially transparent to the measuring light beam (56); b) the thickness of the wafer (12) is calculated from interference signals provided by the measuring interferometer (42); c) the collecting optics (46) are controlled such that a focus (58) of the measuring light beam (56) generated by the collecting optics (42) lies at least temporarily on that boundary surface (13) of the wafer (12) which is further away from the collecting optics (42) in the light path of the measuring light beam (56).

12. Method according to claim 11, characterized in that the thickness is calculated only from those interference signals which differ from a noise signal with an amplitude which is maximum or greater than a predetermined threshold value.

13. Method according to one of claims 11 or 12, characterized in that the collecting optics (46) are controlled such that the interference signals provided by the measuring interferometer (42) are permanently distinguished from a noise signal with an amplitude which is maximum or greater than a predetermined threshold value.

14. Method according to one of claims 11 or 12, characterized in that the collecting optics (46) are controlled such that the focus (58) of the measuring light beam (56) varies periodically.

15. Method according to 14, characterized in that the focus varies according to a sine function.

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