Measuring device and method for measuring thickness of wafer

Abstract

A measuring device for measuring the thickness of a wafer (12) having two parallel interfaces (13, 36) has a measuring interferometer (42) having a light collecting system (46) and a light source (44) for generating a measuring light beam (56). The measurement interferometer (42) directs a measurement light beam (56) through the light collection system (46) towards the wafer (12). The wavelength range of the measuring light beam (56) is selected such that the measuring light beam (56) can at least partially pass through the wafer (12). The measuring device also has an evaluation unit (62) which calculates the thickness of the wafer (12) on the basis of the interference signal provided by the measuring interferometer. According to the invention, the light collecting system (46) is configured to vary the axial position of the focal point of the measurement light beam (56) in dependence on the control signal. A control signal for the optical element (64) is defined in such a way that a focal point (58) of the measurement light beam (56) generated by the light collecting system (46) lies at least temporarily on an interface (13) of the wafer (12) that is farther from the light collecting system (46) in the beam path of the measurement light beam (56).

Description

Background Technology 1. Technical Field

[0002] The present invention relates to a measuring device and a method for measuring wafer thickness, particularly for inline thickness measurement during grinding, polishing or calendering operations that remove material from one side of the wafer.

[0003] 2. Existing Technology

[0004] A wafer typically refers to a circular or square disk with a thickness of approximately 1 mm, used as a substrate for integrated circuits, micromechanical components, or optoelectronic coatings. Semiconductor wafers are usually manufactured from single-crystal or polycrystalline blanks (so-called ingots), which are then sawn transversely along their longitudinal axis into individual wafers. Besides silicon, gallium arsenide, silicon carbide, and indium phosphide are primarily used as semiconductors. Thin sheets made of glass are also sometimes referred to as wafers, for example, in the fabrication of microlens arrays.

[0005] The wafer, referred to as the front side, carries integrated circuits or other functional components such as lenses. After the integrated circuits or other functional components are built, before dicing the wafer and packaging the individual integrated circuits or functional components, the wafer is typically ground, polished, or sintered on the back side to remove unwanted material. Thin wafers have several advantages. For example, thermal resistance is reduced; additionally, problems caused by different coefficients of thermal expansion are reduced. Material removal, also known as back-side grinding or back-side thinning, usually involves several steps using increasingly finer-grit grinding wheels or polishing cloths.

[0006] Currently, the back side of the wafer is ground to a thickness of approximately 300 µm. It is expected that in the near future, the wafer thickness will be only 100 to 150 µm.

[0007] Currently, these values ​​can only be achieved with continuous monitoring of the polishing process. US 2002 / 0048901 A1 proposed optically measuring wafer thickness during the polishing of the wafer's back side.

[0008] DE 10 2011 001 238 A1 discloses a monitoring device having an optical coherence tomography scanner that continuously measures thickness during a polishing operation. This coherence tomography scanner generates a measurement beam composed of broadband coherent light, directed towards the back side of the wafer. An interference signal is detected in the coherence tomography scanner, which is generated by the superposition of two components of the measurement light. The first component consists of the measurement light reflected by the back-side interface of the wafer facing the coherence tomography scanner. The second component consists of the measurement light passing through this back-side interface and reflected by the opposite front-side interface of the wafer, which is provided with functional elements. The wavelength of the measurement light is within the range where the wafer's absorption coefficient is minimal. This results in a sufficiently strong interference signal.

[0009] However, practice has shown that, especially for highly doped wafers, even with appropriate wavelength selection, the reflection from interfaces farther from the coherence tomography head is sometimes weaker than that from interfaces closer to the head, resulting in insufficient interference contrast for reliable measurements.

[0010] The current solution to this problem is to use a measurement head equipped with a light-gathering system and a large depth of field. If the focus is roughly positioned at the center between the two interfaces of the wafer, and the depth of field is large enough, then although the positioning of the measurement head and the wafer inevitably varies, the two interfaces will always be in the region where the measurement light is well focused. This is advantageous because the better the light is focused on the interface, the stronger the reflection and interference signals.

[0011] However, even with a large depth-of-field light-gathering system, interference signals sometimes cannot be fully highlighted from the noise. Summary of the Invention

[0012] Therefore, the object of the present invention is to provide a measuring apparatus with a measuring interferometer for measuring wafer thickness, and a method thereof, wherein the generated interference signal is sufficiently strong even under adverse conditions.

[0013] Regarding the measuring apparatus, the solution of the present invention to achieve the above-mentioned objective is a measuring apparatus for measuring the thickness of a wafer having two parallel interfaces, wherein the measuring apparatus includes a measuring interferometer having a light-collecting system and a light source for generating a measuring beam. The measuring interferometer is configured to direct the measuring beam toward the wafer through the light-collecting system. The wavelength range of the measuring beam is selected such that the measuring beam can at least partially pass through the wafer. An evaluation unit is configured to calculate the thickness of the wafer based on the interference signal provided by the measuring interferometer. According to the present invention, the light-collecting system is configured to change the axial position of the focal point of the measuring beam according to a control signal. The control signal for the light-collecting system can be defined as follows: such that the focal point of the measuring beam generated by the light-collecting system is at least temporarily located on the interface of the wafer that is farther from the light-collecting system in the optical path of the measuring beam.

[0014] As mentioned earlier, the main reason for the weak interference signal is that the reflection at the distant interface is much weaker than the reflection at the near interface. This low interference contrast occurs especially when the wafer has a particularly high absorption rate of the measurement light transmitted into it due to its doping. In the case of such wafers, a large amount of light is absorbed on its way to the distant interface, resulting in weak reflection despite being roughly focused due to the large depth of field.

[0015] This invention proposes to improve interference contrast by increasing the reflection intensity at the distant interface while simultaneously reducing the intensity at the near interface. This is achieved by precisely focusing the measurement beam generated by the light-collecting system, at least temporarily, on the distant interface of the wafer. By precisely focusing on the distant interface, the reflection intensity there is significantly increased. Simultaneously, the beam diameter at the near interface widens, thus correspondingly reducing the reflection intensity there. This effect, i.e., reducing reflection at the near interface, can be significantly enhanced by reducing the depth of field during focusing. For this purpose, for example, a light-collecting system with a high numerical aperture can be used.

[0016] However, the height of the wafer changes during the polishing operation or during the removal of the polishing cloth. Therefore, it is not possible to permanently set the height of the light-collecting system in a single operation so that the focus of the measurement beam is permanently located on the distant interface of the wafer. Therefore, the measuring device must be able to change the axial position of the focus so that the focus is at least temporarily located on the distant interface. For this purpose, the light-collecting system preferably includes a liquid lens or another optical element whose converging or scattering effect can be changed by a control signal. This allows the axial position of the focus to be changed.

[0017] As an alternative to altering the converging or scattering effects of an optical element, the axial position of the focal point can be changed by mechanically moving the optical element, particularly along its axis, using a moving device. The moving device is controlled by a control signal, thus allowing the axial position of the focal point to be altered.

[0018] Adjustable focal length measuring devices are known in the prior art, but are used for other purposes. For example, WO 2021 / 140396 A1 discloses a laser processing apparatus with an optical confocal ranging device, which includes a measuring optical element with a variable focal length. In the case of such known measuring devices, in order to increase the measuring range of the ranging device, it is preferable to periodically adjust the focal length of the measuring optical element.

[0019] The evaluation unit is preferably configured to calculate the thickness only based on the interference signal that stands out from the noise signal with an amplitude of maximum amplitude or greater than a preset threshold. This avoids the influence of weak and therefore unreliable interference signals on the thickness measurement.

[0020] In one embodiment, the focal point of the measurement beam is permanently located at the distant interface of the wafer. This is achieved via autofocus adjustment, configured to define control signals for the light-gathering system in a manner that ensures the interference signal provided by the measurement interferometer always stands out from the noise signal with a maximum amplitude or an amplitude greater than a preset threshold. Therefore, the autofocus adjustment continuously changes the axial position of the focal point by correspondingly controlling the light-gathering system and examining its effect on the amplitude of the interference signal. Once the amplitude reaches its maximum value (or alternatively exceeds the preset threshold), the thickness is calculated based on the interference signal, thereby performing the measurement.

[0021] If the focus of the measurement beam is only temporarily located on the distant interface of the wafer, the adjustment technique is simpler because the focus sweeps across the distant interface intermittently. This can be achieved by a control system configured to define control signals for the light-gathering system in a manner that causes the focus of the measurement beam to change periodically. If the change in focus is large enough, it will sweep across the distant interface of the wafer at regular time intervals. The moment when the focus is exactly on the distant interface can be identified when the interference signal reaches its maximum value. Preferably, the thickness is calculated based on this maximum interference signal, thereby evaluating this maximum interference signal.

[0022] For example, the axial position of the focal spot can vary sinusoidally. This allows for particularly convenient control of the liquid lens. For typical measurement tasks, the period of the sine function can be between 100 and 200 ms. For example, a period duration of 150 ms achieves a measurement frequency of approximately 6 Hz. In this case, six wafers per second can be fed to the measurement device for measurement.

[0023] The evaluation unit can be part of a measurement interferometer. The light-gathering system is typically located in a measurement head fixed near the wafer, which is connected to a more distant benchtop device via fiber optic cables and control lines. In this case, the benchtop device includes, for example, a light source, a detector for the measurement interferometer, an adjustment or control system for the liquid lens, and the evaluation unit. However, the evaluation unit may also be located elsewhere, and particularly implemented in software within a PC, where the interference signal is converted into an electrical signal and then transmitted to the PC.

[0024] Therefore, the present invention also relates to a measurement interferometer without an evaluation unit. The measurement interferometer has a light-collecting system and a light source for generating a measurement beam, wherein the measurement interferometer is configured to direct the measurement beam through the light-collecting system towards a wafer having two parallel interfaces. According to the invention, the light-collecting system is configured to change the axial position of the focal point of the measurement beam according to a control signal. The control signal for the optical element can be defined such that the focal point of the measurement beam generated by the light-collecting system is, at least temporarily, located on the interface of the wafer that is farther from the light-collecting system in the optical path of the measurement beam.

[0025] In terms of methodology, the solution for achieving the objective described at the outset is a method for measuring the thickness of a wafer having two parallel interfaces, wherein the method includes the following steps:

[0026] a) A measurement interferometer generates a measurement beam and directs the measurement beam toward a wafer via a light-collecting system, wherein the wavelength range of the measurement beam is selected such that the measurement beam can at least partially pass through the wafer;

[0027] b) Calculate the thickness of the wafer based on the interference signal provided by the measuring interferometer;

[0028] c) Control the light-collecting system so that the focal point of the measurement beam generated by the light-collecting system is at least temporarily located on the interface of the wafer that is farther from the light-collecting system in the optical path of the measurement beam.

[0029] In terms of methodology, the explanation of advantageous technical solutions above also applies accordingly. Attached Figure Description

[0030] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Wherein:

[0031] Figure 1 A polishing apparatus for back-side polishing of a processed wafer is shown, having a measuring apparatus of the present invention for measuring wafer thickness;

[0032] Figure 2 The diagram schematically illustrates the axial position of the focal point of the measuring beam generated by a conventional measuring device;

[0033] Figure 3a The diagram schematically illustrates the axial position of the focal point of the measuring beam generated by the measuring device of the present invention;

[0034] Figure 3b This schematically illustrates how the axial position of the measurement beam focus is reguided in the present invention when the position of the wafer relative to the measuring device changes;

[0035] Figure 4 for Figure 1 A partial view of the measuring device shown, which periodically changes the position of the focal point;

[0036] Figure 5 The focal axis position as a function of time is shown in a graph; and

[0037] Figure 6 The intensity of the interference signal as a function of time is shown in a graph. Detailed Implementation

[0038] 1. Measuring the wafer

[0039] Figure 1 A polishing apparatus, generally indicated by 10, is shown for back-side polishing of a processed wafer 12, and a measuring device 14 of the present invention is shown for measuring the thickness of the polished wafer 12.

[0040] The polishing apparatus 10 includes a disk-shaped wafer holder 16 with a plurality of grooves 18 distributed on its circumference. A wafer 12 can be placed in each groove 18. The wafer 12 may be, for example, a pre-processed silicon wafer, and its... Figure 1 Electronic circuitry (not shown) is mounted on the front side 13, facing downwards. The wafer holder 16 can be rotated about a rotation axis 22 by means of a drive device 20 to transport the wafer 12 to several stations distributed around the circumference, as shown in the cross-sectional view. Figure 1 Only one of these workstations, measurement workstation 23, is shown in the image.

[0041] A sandpaper holder 24 is provided opposite to the groove 18, and a sandpaper 26 is replaceably fixed on its bottom side. During the polishing process, the sandpaper holder 24 is rotated around the rotation axis 30 by means of another drive device 28, thereby causing the sandpaper 26 to move on the wafer 12 and remove material from the back side 36 of the wafer.

[0042] A casing 32 is provided at the measuring station 23 to prevent the polishing slurry 34 from flowing out. The polishing slurry 34 is located between the protective glass 38 and the back surface 36 of the anti-recession groove 18 of the wafer 12, which extends parallel to the front surface 13 of the wafer 12.

[0043] The measuring device 14 measures the thickness of the wafer 12 located at the measuring station 23 at a given time point. The measuring device 14 includes a measuring interferometer 42, which has a light source 44, a light collection system 46, and a detector 48. The light source 44 generates broadband measurement light, which is transmitted to the light collection system 46 via an optical fiber 50. In this way, the latter is separated from the rest of the measuring device 14 and constitutes a compact measuring head 52 containing its own housing 54, allowing for a space-saving arrangement even in confined spaces such as the measuring station 23. The remaining components of the measuring device 14 can be arranged as follows: Figure 1 As schematically shown, they are all housed in the desktop device housing 55, but can also be distributed across different individual devices. These remaining components mainly include a central control module 61 with an operating unit and an evaluation unit 62.

[0044] The light-collecting system 46 focuses the measurement light emitted from the optical fiber 50 to form a measurement beam 56, which penetrates the protective glass 38 and the polishing slurry 34 and converges at the focal point 58. A portion of the measurement light incident on the wafer 12 is reflected on the back surface 36 of the wafer 12. The remaining measurement light penetrates the wafer 12 and is partially absorbed there. The unabsorbed portion of the penetrating measurement light reaches the front surface 13 of the wafer 12 and is partially reflected there. These two reflected measurement light components return to the housing 55 through the light-collecting system 46 and the optical fiber 50, and are then transmitted to the detector 48 via the beam splitter 60. This detector is configured as a spectrometer and detects the interference signal generated by the superposition of these two reflected measurement light components. The intensity of the interference signal is detected according to the wavelength and converted into an electrical signal by the detector 48. The evaluation unit 62 calculates the thickness of the wafer 12 at the incident position of the measurement beam 56 based on these electrical signals. This measurement method is called 1D-se FD OCT (one-dimensional spatially encoded Fourier Domain Optical Coherence Tomography). Alternatively, 1D-te FDOCT (one-dimensional time-encoded Fourier Domain Optical Coherence Tomography) can be used when an adjustable light source is required.

[0045] The calculated thickness assessment is transmitted to the machine control system 63, which controls the drive units 20 and 28 for the wafer holder 16 and the abrasive cloth holder 24. If the wafer 12 reaches the desired nominal thickness, the polishing operation is terminated.

[0046] In this regard, the grinding device 10 and the measuring device 14 are known, and therefore will not be described in detail.

[0047] The light-collecting system 46 of the measuring device 14 preferably includes a liquid lens 64, the focal length of which can be changed by an electrical control signal. The liquid lens 64 is configured as a condensing lens, but it can also have a scattering effect provided the other lenses in the light-collecting system 46 have a sufficiently strong converging effect. The liquid lens 64 receives electrical control signals from a control system 65, which is also located in the housing 55 and may be part of a central control module 61. The axial position of the focal point 58 can be changed within certain limits using the liquid lens 64. According to the invention, the focal point 58 is axially positioned such that the focal point is at least temporarily located on the front surface 13 of the wafer 12.

[0048] The following will refer to Figure 2 , Figure 3a and Figure 3b Explain the advantages that result from this.

[0049] Figure 2 The axial position of the focal point 58, fixed in a conventional measuring apparatus, is shown. The focal point 58 is located between the front surface 13 and the back surface 36 of the wafer 12. Due to the weak convergence of the measuring beam 56, the depth of field is relatively large. Therefore, the measuring light covers a small area on both the front surface 13 and the back surface 36 of the wafer 12. The positions of the front surface 13 and the back surface 36 are as follows... Figure 2 When changes are made as shown by the dashed line, the area covered does not change significantly due to the large depth of field.

[0050] However, even with optimized wavelength selection, if the wafer 12 still absorbs the measurement beam 56 significantly due to its doping, the reflection on the back side 36 may be much higher than the reflection on the more distant front side 13, resulting in excessively low interference contrast. Consequently, the amplitude of the interference signal after Fourier transform is insufficient to stand out from the noise.

[0051] Figure 3a The focal point 58 is shown to be precisely located on the far front side 13 of the wafer 12 according to the invention. By focusing the measurement beam 56 onto the far front side 13 of the wafer 12, which is farther from the light-collecting system 46, a very small and intense spot is formed there, resulting in correspondingly strong reflection. The numerical aperture of the light-collecting system 46 is larger than that of conventional light-collecting systems, thus resulting in a smaller depth of field. Therefore, the measurement light is distributed over a larger area on the back side 36 of the wafer 12, resulting in reduced reflection. These two effects significantly improve the interference contrast and the amplitude of the interference signal.

[0052] If the axial positions of front 13 and back 36 are as follows Figure 3b If the change is as shown in the diagram for chip 12', the focal point 58 of the measurement beam 56, indicated by the dashed line, is no longer located on the front surface 13. Therefore, the light-collecting system 46 must reguide this focal point using the liquid lens 64. Subsequently, the reguided focal point 58' of the measurement beam 56' is accurately located back on the front surface 13, so that the amplitude of the interference signal does not decrease or does not decrease significantly.

[0053] The autofocus adjustment ensures that the focus 58 is always on the front side 13 of the crystal 12. To do this, the autofocus adjustment evaluates the amplitude of the interference signal and changes the focal length of the liquid lens 64 until the amplitude reaches its maximum.

[0054] like Figure 4As shown, a simpler solution is to periodically change the axial position of the focus 58 according to a predetermined periodic function. Several axially closely aligned focuses 58' are shown in the figure, generated sequentially by continuously adjusting the liquid lens 64. At a specific point in time, one of the focuses 58' must be located on the front side 13 of the wafer 12, which can be identified by the maximum amplitude of the interference signal. Only this signal is further evaluated to calculate the thickness of the wafer 12.

[0055] Figure 5 The axial position z of focus 58 is shown in the graph. focus How the focal length changes according to a sine function over time t. The axial position of the front surface 13 of wafer 12 is indicated by dashed line 68. At time points t1, t2, and t3, the focal point 58 is located on the front surface 13, thus measurements can be performed. Deadlock times 70a and 70b are indicated by rectangles, during which measurements cannot be performed because a new wafer 12 is being inserted into the measurement station 23. The period of focal length change is determined such that, within the available time period between deadlock times 70a and 70b, the focal point 58 sweeps across the front surface 13 at least once.

[0056] Figure 5 The graph schematically illustrates the interference signal superimposed across all frequencies as it varies with time t. The amplitude of the interference signal is largest at time points t1, t2, and t3 when focus 58 is located on the front face 13, thus measurements can be performed at these time points. 70 schematically represents a noise level from which the interference signal must be reliably distinguished.

Claims

1. A measuring apparatus for measuring the thickness of a wafer (12) having two parallel interfaces (13, 36), comprising: A measuring interferometer (42) having a light-collecting system (46) and a light source (44) for generating a measuring beam (56), wherein, The measurement interferometer (42) is configured to direct the measurement beam (56) toward the wafer (12) through the light-collecting system (46), wherein, when selecting the wavelength range of the measurement beam (56), the measurement beam (56) is allowed to at least partially pass through the wafer (12), and has Evaluation unit (62), configured to calculate the thickness of the wafer (12) based on the interference signal provided by the measuring interferometer, Its features are, The light-collecting system (46) is configured to change the axial position of the focal point of the measurement beam (56) according to a control signal, and The control signal can be defined in a way that the focal point (58) of the measurement beam (56) generated by the light collection system (46) is at least temporarily located on the interface (13) of the wafer (12) that is farther from the light collection system (46) in the optical path of the measurement beam (56).

2. The measuring device according to claim 1, characterized in that, The light-gathering system (46) includes an optical element (64) whose converging or scattering effect can be changed by the control signal.

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

4. The measuring device according to any one of the preceding claims, characterized in that, The evaluation unit (62) is configured to calculate the thickness only based on the interference signal that stands out from the noise signal with the maximum amplitude or an amplitude greater than a preset threshold.

5. The measuring device according to any one of the preceding claims, characterized in that... The autofocus adjustment is configured to define the control signal in a certain way, such that the interference signal provided by the measuring interferometer (42) always stands out from the noise signal with a maximum amplitude or an amplitude greater than a preset threshold.

6. The measuring device according to any one of claims 1 to 5, characterized in that... The control system (65) is configured to define the control signal in such a way that the focus (58) of the measurement beam (56) changes periodically.

7. The measuring device according to claim 6, characterized in that, The focal point (58) changes according to the sine function.

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

9. A measuring interferometer (42) having a light-collecting system (46) and a light source (44) for generating a measuring beam (56), wherein, The measurement interferometer (42) is configured to direct the measurement beam (56) through the light collection system (46) to a wafer (12) having two parallel interfaces (13, 36). Its features are, The light-collecting system (46) is configured to change the axial position of the focal point of the measurement beam (56) according to a control signal, and The control signal can be defined in a way that the focal point (58) of the measurement beam (56) generated by the light collection system (46) is at least temporarily located on the interface (13) of the wafer (12) that is farther from the light collection system (46) in the optical path of the measurement beam (56).

10. The measuring interferometer according to claim 9, characterized in that, The light-gathering system (46) includes an optical element (64) whose converging 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) The measuring interferometer (42) generates a measuring beam (56), and The measurement beam (56) is directed to the wafer (12) through the light-collecting system (42), wherein, When selecting the wavelength range of the measurement beam (56), such that The measuring beam (56) is able to pass through the chip (12) at least partially. b) Calculate the thickness of the wafer (12) based on the interference signal provided by the measuring interferometer (42); c) Control the light-collecting system (46) such that the focal point (58) of the measurement beam (56) generated by the light-collecting system (42) is at least temporarily located on the interface (13) of the wafer (12) that is farther from the light-collecting system (42) in the optical path of the measurement beam (56).

12. The method according to claim 11, characterized in that, The thickness is calculated solely based on the interference signal that stands out from the noise signal with an amplitude of maximum amplitude or greater than a preset threshold.

13. The method according to any one of claims 11 or 12, characterized in that, The light collection system (46) is controlled so that the interference signal provided by the measuring interferometer (42) always stands out from the noise signal with the maximum amplitude or an amplitude greater than a preset threshold.

14. The method according to any one of claims 11 or 12, characterized in that, The light-collecting system (46) is controlled so that the focal point (58) of the measurement beam (56) changes periodically.

15. The method according to 14, characterized in that, The focal point varies according to the sine function.

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