high precision vapor phase decomposition-drop collection (VPD-DC) scanning

Abstract

The present invention relates to a method and system for performing vapor phase decomposition-droplet collection (VPD-DC) on the surface of a wafer, wherein a pipette replaces the function of a scanning tube and is operated such that a bulge of scanning liquid protrudes from the channel of the pipette and contacts the surface of the wafer for scanning.

Description

Technical Field

[0001] This invention generally relates to wafer processing, and more specifically to the analysis of impurities in wafers. Background Technology

[0002] The development of the semiconductor industry has placed stringent requirements on the purity of wafers, such as silicon wafers, with specifications often exceeding the sensitivity of conventional analytical techniques. VPD-DC (vapor phase decomposition-drop collection) is a sample preparation method used to concentrate impurities on the surface of the wafer to improve the sensitivity of subsequent analytical processes.

[0003] In the first step of VPD-DC, the surface oxides of the wafer are etched in an HF (hydrogen fluoride) atmosphere. Subsequently, a small droplet is scanned across the wafer surface to collect and pre-concentrate surface impurities within the droplet. Wet chemical analysis can then be performed on the sample using, for example, inductively coupled plasma mass spectrometry (ICPMS) or atomic absorption spectrometry (AAS). Alternatively, the droplet is dried for analysis of the dried sample, such as total reflectance X-ray fluorescence analysis (TXRF).

[0004] In practice, a major improvement in handling this time-consuming and labor-intensive process is the development of fully automated systems that allow for cassette-to-cassette VPD-DC processing. These systems comprise two or three processing modules adjacent to the cassette station and a robotic module for wafer transfer, housing etching, scanning, and optional drying processes. The scanning module typically includes a rotating wafer stage or scanning stage, a pipette for droplet deposition, and a scanning tube for capturing and guiding the droplets onto the wafer surface. Once the freshly etched wafer is placed on the scanning stage, aligned, and its plane / notch oriented, the scanning process is initiated. The pipette deposits droplets of scanning liquid onto the wafer surface. The scanning tube then picks up the droplets and, if appropriate, guides them across the wafer surface in a spiral trajectory or a circular trajectory that only covers the surface. After scanning is complete, the pipette again picks up the scanning liquid now carrying the solute and transfers it for further analysis.

[0005] In a variant of VPD-DC, known as bevel scanning, the scanning liquid is guided only along the edge / bevel of the wafer in a circular trajectory. In this variant, the droplet is not deposited on the wafer surface, but rather on a droplet holder in the form of a small, concave platform positioned below the circumferential edge of the wafer. The droplet is then held between the scan tube and the droplet holder, and as the wafer rotates on the scan stage, the wafer edge contacts the held droplet laterally.

[0006] Due to the inherent minimum size and deformability of droplets, precise scanning of very narrow strips, such as a strip only 1 mm wide on an upper sloping surface, has so far been impractically impossible. However, the ability to scan very precise and narrow strips and regions has attracted interest in some applications. Summary of the Invention

[0007] The present invention aims to provide a system and method that allows for scanning very precise and narrow strips and regions.

[0008] In this context, the present invention relates to a method for performing VPD-DC on the surface of a wafer, the method comprising the steps of: a) placing an etched wafer on a rotatable scanning stage; b) positioning a pipette and the wafer relative to each other such that the tip of the pipette is close to, but not in contact with, the surface of the wafer; c) manipulating the pipette such that a bulge of scanning liquid protrudes from the pipette channel and contacts the surface of the wafer; and d) rotating the wafer to guide the bulge of scanning liquid protruding from the pipette channel along the surface of the wafer.

[0009] Therefore, compared to existing technologies, this invention does not deposit droplets of scanning liquid on the wafer surface, or on the droplet holder in the case of bevel scanning, and does not use a scanning tube to guide the droplets across the wafer surface or along the wafer edge. More precisely, this invention relies solely on a liquid bulge protruding from but never leaving the tip of the pipette, and this liquid bulge does not deposit on the wafer surface or the droplet holder. A separate scanning lever is no longer needed, as the pipette already functions as the scanning lever.

[0010] Compared to a complete droplet, relying solely on a liquid bulge protruding from the tip of the pipette allows for very fine control of the scanning area and reduces the minimum width of the scannable stripes. For example, this method allows for scanning very fine stripes, such as stripes less than 2 mm or even less than 1 mm wide on the upper bevel of a wafer, which is desirable in some applications. In some embodiments, the invention enables analysis to be performed using a much smaller liquid volume, as in previous droplet-based methods, because there is no inherent lower limit to the liquid volume applied by a droplet.

[0011] The wafer can be a silicon wafer, but the invention can also be applied to any other type of wafer.

[0012] Generally, the scanning liquid for VPD-DC is an aqueous solution of HF and H2O2.

[0013] In one embodiment, the method is a bevel scanning VPD-DC. In this embodiment, in step b), the pipette and the wafer are positioned relative to each other such that the tip of the pipette is close to, but not in contact with, the bevel of the wafer. In step d), the wafer is rotated to guide the bulge of scanning liquid protruding from the pipette channel along the bevel of the wafer.

[0014] Typically, especially for bevel scanning VPD-DC, it is preferable that the tip of the pipette has a bevel to form an angled pipette channel opening. This angle approximately corresponds to the angle of the wafer bevel. This results in improved wetting of the wafer bevel within the inventive concept.

[0015] In other embodiments, particularly if it is desirable to scan all or part of the flat surface of the wafer, the tip of the pipette is preferably uniformly flat.

[0016] Using only the bulge protruding from the pipette for the scanning process requires extremely high precision in the relative positioning of the pipette tip and the wafer.

[0017] To this end, in addition to coarse alignment of the wafer on a scanning stage using a line scan camera, the method includes improved alignment using a high-sensitivity camera on the scanning stage. This allows for determining the position of the wafer edge in the xy plane or the tip of the pipette in the xz plane with an accuracy of less than 10 μm, preferably less than 8 μm, or even less than 5 μm. This high-sensitivity camera is preferably a 2D camera. The method can also use such a high-sensitivity camera to fine-tune the relative position of the pipette tip and the wafer edge / bevel. For any fine-tuning of the relative position, a shuttle system using a scanning stage is preferred over a robotic arm system because the shuttle system achieves higher accuracy during movement.

[0018] In one implementation, a highly sensitive camera can be used to monitor the contour of the bulge. The photograph of this contour can be analyzed to determine the success rate of the scan, for example, to determine if there is tearing in the liquid film formed between the bulge and the wafer surface. In addition to indicating whether the scan was successful, the photograph of the contour also conveys information about whether the volume of the bulge changed during the scan.

[0019] Furthermore, since wafers can be imperfect and not ideally spherical, to further improve accuracy, especially during bevel scanning, the method includes continuously adjusting the relative positions of the scanning stage and the tip of the pipette during scanning, based on pre-recorded images from a high-sensitivity camera. For this continuous adjustment process, a shuttle system for the scanning stage is also preferred.

[0020] In one implementation, the volume of the bulge can be adjusted during scanning to account for a hypothetical volume reduction of the bulge due to evaporation of liquid, or a hypothetical volume increase of the bulge due to increased humidity. In the case of monitoring the contour of the bulge, as previously described, the volume of the bulge can be adjusted based on information extracted from photographs taken during scanning to account for an actual decrease or increase in the volume of the bulge.

[0021] In the context of the initial description, the present invention also relates to a system for performing VPD-DC on the surface of a wafer, the system comprising a scanning module including: a rotatable scanning stage for placing an etched wafer thereon; a pipette for contacting a scanning liquid with an inclined surface of the wafer placed on the scanning stage; and a control unit configured to operate the scanning stage and the pipette to perform the method of the present invention.

[0022] In addition to the scanning module, the system may include an etching module, and preferably a drying module. Typically, the system also includes a cassette workstation. A robot for wafer transfer may be shared among the modules, or the system may include two or more robots (or robotic arms). The system may also include devices for wet probe analysis or dry probe analysis, such as inductively coupled plasma mass spectrometry (ICPMS) or atomic absorption spectrometry (AAS), and dry probe analysis such as total reflectance X-ray fluorescence analysis (TXRF). The system is preferably fully automated and includes a common control unit operatively connected to all components of the system.

[0023] A scanning stage is typically associated with a shuttle for moving the scanning stage in a horizontal plane. Similarly, the system typically includes a robotic arm for manipulating a pipette and moving it in three-dimensional space or in a plane with a vertical component.

[0024] As described in conjunction with the method of the present invention, in some embodiments, it is preferred that the tip of the pipette has an angle to form an angled pipette channel opening, while in other embodiments, the tip of the pipette may be uniformly flat.

[0025] In one embodiment, the system may include a high-precision camera, preferably a 2D camera that allows the position of the edge of the wafer in the xy plane or the tip of a pipette in the xz plane to be determined with an accuracy of less than 10 μm, preferably less than 8 μm, or even less than 5 μm. When in a scanning position, the camera's viewing direction is preferably horizontal and along the tangent of the wafer.

[0026] The system may also include a droplet holder, preferably in the form of a small, concave platform. When in the scanning position, the droplet holder may be positioned below the peripheral edge line of the wafer and / or below the tip of the pipette. The droplet holder is used to retain a volume of scanning liquid before it is drawn into the pipette for scanning and to collect the scanning liquid after scanning. Attached Figure Description

[0027] Further details and advantages of the invention are described below with reference to non-limiting examples and exemplary drawings. The drawings show:

[0028] Figure 1 The edge and beveled surface of the wafer to be scanned in the method according to the invention;

[0029] Figure 2 Side view of the tip of the pipette and the edge of the wafer during positioning;

[0030] Figure 3 Side view of the tip of a pipette with a protruding bulge of scanning liquid in contact with the bevel of the wafer during scanning. Detailed Implementation

[0031] This example illustrates the high-precision upper bevel scanning method and system of the present invention for detecting surface contamination on wafers. Figure 1 A cross-sectional view of the edge portion of wafer 100 is shown, in which a slope extending between point A and point B and having a width x typically of about 1 mm can be identified.

[0032] Initially, the method includes a step of coarsely aligning the wafer on a scanning stage. More specifically, this step involves rotating the scanning stage on which the wafer is mounted and taking pictures of the wafer's edges using a line scan camera. The system is configured to take one picture every 90 degrees of wafer rotation. The deviation between the wafer's edge positions in the four pictures is used to calculate a better position for the wafer on the scanning stage. To reach the calculated improved position, a robot lifts the wafer and corrects for the deviations in the x and y directions. The results are then controlled again by the linear camera if necessary, and this process is repeated. The coarse alignment step of the wafer is completed once the deviation between the farthest wafer edge positions is detected to reach a target accuracy of less than 200 μm. A maximum number of iterations may also exist, for example, up to four iterations before the step terminates.

[0033] Following the coarse localization, fine localization is performed using a 2D camera with a very good resolution of less than 70 μm. The 2D camera's field of view is approximately horizontal and along the tangent of the wafer. The fine localization step involves moving the scanning stage and wafer to a so-called inclined position using the shuttle of a scanning stage operating in the xy plane. In the inclined position, the edge of the wafer is positioned above the droplet holder. The 2D camera then begins capturing images of the wafer edge while the wafer is rotated 450 degrees. The captured images are analyzed to calculate the improved position of the wafer on the scanning stage. The method for reaching the improved position can be substantially described with reference to the steps described above, with the only difference being a target deviation of 80 μm.

[0034] Following precise positioning is the notch positioning step, which involves rotating the scanning stage until the 2D camera detects the notch on the wafer (the notch is pre-fabricated into the wafer before performing the bevel scanning VPD-DC of this invention). Once the notch location is identified, the scanning stage rotates until the notch moves to a predefined (angle) starting position.

[0035] In the next step, the (double) pipette with beveled tips and the wafer are positioned relative to each other. Figure 2 A side view of the tip 125 of the pipette 120 and the bevel 104 of the wafer 100 during this step, as observed from the perspective of a 2D camera, is shown. For this purpose, the wafer 100 and the pipette 120 are moved to a defined initial position where the bevel position (wafer) and the operating position (pipette) are close to each other. To handle the pipette 120, the system includes a robotic arm. The 2D camera is then used to capture the actual relative positions of the bevel 104 of the wafer and the tip 125 of the pipette. Based on the photograph, the system calculates the distances between the bevel 104 of the wafer and the tip 125 of the pipette in the x and z directions. The measured distances are then compared with the defined distances between the bevel 104 of the wafer and the tip 125 of the pipette in the x and z directions. The resulting deviation is converted into motor steps and fed back to the motors of the robotic arm used to move the pipette and the shuttle of the scanning stage. In response to this information, the robotic arm and the shuttle of the scanning stage move in the defined directions to approach the desired position. The camera then captures another image, which is evaluated again to confirm that there is an appropriate distance in the x and z directions between the bevel 104 of the wafer and the tip 125 of the pipette. If the position is still not appropriate, this process is repeated. If it is appropriate, the image is stored as a reference for the position of the wafer 100 and the pipette 120 during subsequent scans.

[0036] Next, pipette 120 is used to transfer a predefined volume (e.g., 50 μl–200 μl) of fresh scan solution to the droplet holder. A rinsing and calibration procedure is then performed, with liquid aspirated again and a smaller portion ejected to avoid any problems caused by recoil from changes in orientation. Pipette 120 is then moved to the defined scan position.

[0037] In order to perform the scanning process, a small volume of liquid, such as 1 μl, is squeezed out from the tip 120 of the pipette, so that a small bulge 130 of the scanning liquid protrudes from the channel of the pipette and contacts the surface of the wafer. Figure 3 A side view of the tip 125 of the pipette during this step is shown, where a protruding bulge 130 of the scanning liquid contacts the bevel 104 of the wafer. The wafer 100 is then rotated less than 360 degrees, for example, 355 degrees, to avoid scanning notches. After scanning, the stage moves back to its default position, and the pipette 120 dispenses all the liquid contained in the pipette onto a droplet holder.

[0038] The volume of the bulge can be adjusted during the scan to account for a hypothetical volume reduction in the bulge due to evaporation of liquid, or a hypothetical volume increase in the bulge due to increased humidity. For example, an additional volume equivalent to 20% of the initial bulge volume can be added for every 90 degrees of scanning. The addition or reduction of the bulge volume can follow a predetermined manner, which can be default or based on previous experiments in a given environment.

[0039] During scanning, the outline or length of the bulge of the droplet protruding from the pipette channel can be monitored using a 2D camera. The photographs can be analyzed to determine the success rate of the scan, for example, to determine if there is tearing in the liquid film used to attach the bulge to the wafer surface. Numerically, in one embodiment, the length of the bulge extending from the pipette channel to the wafer surface / bevel should preferably be kept within the range of 1 mm ± 1 μm. Besides indicating whether the scan was successful, the photographs also convey information about whether the volume of the bulge changed during the scan.

[0040] In the context of monitoring the outline of a bulge, monitoring can also be used to adjust the volume of the bulge based on information extracted from photographs taken during scanning, taking into account whether the actual volume of the bulge has decreased or increased.

[0041] After scanning, pipette 120 transfers the liquid containing dissolved impurities from the droplet holder to a vial. This liquid can be diluted using, for example, 650 μm water (1:4 ratio) or another scanning liquid, and is then further processed. Wafer 100 can be transferred back to the assembly or to another workstation. The pipette is self-cleaning and can be used to clean the droplet holder with a cleaning solution.

Claims

1. A method for performing vapor phase decomposition-droplet collection (VPD-DC) on the surface of a wafer, comprising the following steps: a) Place the etched wafer on a rotatable scanning stage; b) Position the pipette and the wafer relative to each other such that the tip of the pipette is close to the surface of the wafer but does not contact the surface of the wafer; c) Operate the pipette such that a bulge of scanning liquid protrudes from the channel of the pipette and contacts the surface of the wafer; as well as d) Rotate the wafer to guide the bulge of the scanning liquid protruding from the channel of the pipette along the surface of the wafer; The method is a slope scanning vapor phase decomposition-drop collection (VPD-DC), wherein in step b), the pipette and the wafer are positioned relative to each other such that the tip of the pipette is close to the slope of the wafer but does not contact the slope of the wafer, and wherein in step d), the wafer is rotated to guide the bulge of the scanning liquid protruding from the channel of the pipette along the slope of the wafer; The method includes using a camera to fine-tune the relative positions of the tip of the pipette in the xz plane and the bevel of the wafer in the xy plane, wherein the camera is a camera that allows the position of the edge of the wafer and the tip of the pipette to be determined with an accuracy of less than 10µm.

2. The method of claim 1, wherein, The method includes using a camera to center the wafer on a horizontal scanning stage, the camera allowing the position of the wafer's edge in the xy plane to be determined with an accuracy of less than 10 µm.

3. The method of claim 1 or 2, wherein, The method includes continuously adjusting the position of the scanning stage during the scanning process in step d) based on pre-recorded photographs from a camera, wherein the camera is a camera that allows the position of the wafer edge to be determined with an accuracy of less than 10 µm.

4. The method of claim 1 or 2, wherein, The method includes using a camera to monitor the contour of the bulge during scanning, the camera being a camera that allows the contour of the bulge to be determined with an accuracy of less than 10 µm.

5. The method of claim 1 or 2, wherein, During scanning step d), the volume of the bulge is adjusted to account for a decrease or increase in the volume of the bulge caused by evaporation or increased humidity.

6. A system for performing vapor phase decomposition-droplet collection (VPD-DC) on the surface of a wafer, the system comprising a scanning module, the scanning module including: A rotatable scanning stage for placing an etched wafer thereon; A pipette for bringing scanning liquid into contact with the bevel of the wafer placed on the scanning stage; The camera is one that allows the position of the edge of the wafer in the xy plane or the tip of the pipette in the xz plane to be determined with an accuracy of less than 10µm; A control unit configured to operate the scanning stage and the pipette to perform the method according to any one of claims 1-5.

7. The system of claim 6, wherein, The tip of the pipette has an angled tip to form an angled pipette channel opening.

Images