HIGH PRECISION VPD-DC SCAN

Abstract

The invention relates to a method and system for performing VPD-DC on wafer surfaces, wherein the pipette substitutes for the function of the scan tube and is operated such that a bulge of scanning liquid protrudes from the pipette channel and contacts the wafer surface for scanning.

Claims

1. A method of performing VPD-DC on wafer surfaces, comprising the steps of: a) placing an etched wafer on a rotatable scan table; b) positioning a pipette and the wafer relative to each other such that the pipette tip is in close proximity, but not in contact with the wafer surface; c) operating the pipette such that a bulge of scanning liquid protrudes from the pipette channel and contacts the wafer surface; and d) rotating the wafer to guide the scanning liquid bulge protruding from the pipette channel along the wafer surface.

2. The method of claim 1, wherein the method is a bevel scan VPD-DC, wherein in step b) the pipette and the wafer are positioned relative to each other such that the pipette tip is in close proximity but not in contact with the wafer bevel, and wherein in step d) the wafer is rotated to guide the scanning liquid bulge protruding from the pipette channel along the wafer bevel.

3. The method of claim 1, wherein the method comprises centering the wafer on the horizontal scan table using a 2D camera which allows determination of the position of the wafer edge in xy plane with an accuracy of below 10 μm.

4. The method of claim 2, wherein the method comprises fine-tuning the relative positions of the pipette tip in xz plane and the wafer bevel in xy plane using a camera, which allows determination of the position of the wafer edge and the pipette tip with an accuracy of below 10 μm.

5. The method of claim 1, wherein the method comprises an ongoing adaption of the position of the scan table during the scan of step d), based on pre-recorded pictures from a camera, which allows determination of the position of the wafer edge with an accuracy of below 10 μm.

6. The method of claim 1, wherein the method comprises monitoring the bulge contour during the scan using a camera which allows determination of the bulge contour with an accuracy of below 10 μm.

7. The method of claim 1, wherein during the scanning step d) the volume of the bulge is adjusted to account for an assumed or actual volume decrease or increase of the liquid bulge due to evaporation.

8. A system for performing VPD-DC on wafer surfaces, the system comprising a scanning module, the scanning module comprising: a rotable scan table for placing an etched wafer thereon; a pipette for bringing a scanning liquid into contact with the bevel of a wafer placed on the scan table; and a control unit configured to operate the scan table and the pipette such as to carry out a method of claim 1.

9. The system of claim 8, wherein the pipette tip is bevelled to create an angled pipette channel opening.

10. The system of claim 8, further comprising a high precision camera, which allows determination of the position of the wafer edge in an xy plane or the pipette tip in an xz plane with an accuracy of below 10 μm.

11. The method of claim 3, wherein the accuracy is below 8 μm.

12. The method of claim 4, wherein the camera is a 2D camera.

13. The method of claim 5, wherein the camera is a 2D camera.

14. The method of claim 5, wherein the accuracy is below 8 μm.

15. The method of claim 5, wherein the camera is a 2D camera and wherein the accuracy is below 8 μm.

16. The method of claim 6, wherein the accuracy is below 8 μm.

17. The method of claim 6, wherein the camera is a 2D camera.

18. The system of claim 10, wherein the camera is a 2D camera.

19. The system of claim 10, wherein the accacury is below 8 μm.

20. The system of claim 10, wherein the accacury is below 5 μm.

Description

[0027] Further details and advantages of the invention are described in the following with reference to a non-limiting example and illustrative figures. The figures show:

[0028] FIG. 1: an edge of a wafer and a bevel surface to be scanned in a method according to the invention;

[0029] FIG. 2: a side view of the pipette tip and the wafer edge during positioning; and

[0030] FIG. 3: a side view of the pipette tip with a protruding bulge of scanning liquid touching the wafer bevel during scan.

[0031] The example shows a high precision upper bevel scan process and system of the invention to detect surface contamination in wafers. FIG. 1 illustrates a cut view on the edge portion of a wafer 100, where the bevel extending between points A and B and having a width of x, usually around 1 mm, can be recognized.

[0032] Initially, the method comprises a step of gross wafer centering on the scan table. In more detail, this step comprises rotating the scan table with the wafer loaded thereon and taking pictures of the wafer edge using a line-scan camera. The system is configured such that a picture is taken every 90° of wafer rotation. The deviations between the wafer edge positions of the four pictures are used to calculate a better position for the wafer on the scan table. To implement the calculated improved position, a robot lifts the wafer and corrects the deviation in x and y direction. The result is controlled again by the line camera and the process repeated if necessary. Once a target accuracy of smaller 200 μm deviation between the most removed wafer edge positions is detected, the gross wafer centering step is completed. There can also be a maximum of iterations, e.g. four iterations, before the step is aborted.

[0033] The gross positioning is followed by a fine positioning using a 2D camera with very good resolution of below 70 μm, whose view direction is approximately horizontally and along a tangential line of the wafer. The fine positioning step comprises moving the scan table and the wafer to a so-called bevel position, using a scan table shuttle operating in xy-plane. In the bevel position, the wafer edge is positioned above a droplet holder. The 2D camera then starts capturing a picture of the wafer edge and the wafer rotates for 450°. The captured pictures are analysed to calculate an improved position of the wafer on the scan table. The implementation can essentially be as described in connection with the foregoing step, with the only difference that the target deviation is 80 μm.

[0034] The fine positioning is followed by a step of notch positioning, which includes rotating the scan table until the 2D camera finds the notch of the wafer (which is prefabricated into the wafer before the bevel scanning VPD-DC of the invention is performed). Once the notch position is identified, the scan table is rotated until the notch moves to a predefined (angular) start position.

[0035] The bevelled tip (dual) pipette and the wafer are subsequently, in a next step, positioned relative to each other. FIG. 2 shows a side view, as viewed from the perspective of the 2D camera, of the tip 125 of the pipette 120 and the bevel 104 of the wafer 100 during this step. For this purpose, the wafer 100 and the pipette 120 are moved to defined raw positions approximating bevel position (wafer) and operative position (pipette). For handling the pipette 120, the system comprises a robot arm. The 2D camera is then used to capture the actual relative positions of the wafer bevel 104 and the pipette tip 125. Based on the picture, the system calculates the distance between wafer bevel 104 and pipette tip 125 in x and z direction. It then compares this measured distance with a defined distance between the pipette tip 125 and wafer bevel 104 in x and z direction. The deviation is converted into motor steps and feedback is given to the motor of the pipette robot arm and the scan table shuttle. The arm and the scan table shuttle, in response to this information, moves in defined directions to approximate the position required. The camera then captures a picture again, which is evaluated again to confirm that there is an appropriate distance between the pipette tip 125 and the wafer bevel 104 in x and z direction. If the position is not yet appropriate, the process is repeated. If appropriate, the pictures is stored as a reference for the position of wafer 100 and pipette 120 during the subsequent scan.

[0036] Next, the pipette 120 is used to transfer a predefined amount of fresh scanning liquid, e.g. 50-200 μl, to the droplet holder. It then carries out a rinsing and calibration routine, sucks in the liquid again, and spits back a smaller part of the liquid to avoid any problems resulting from backlash due to a direction change. The pipette 120 is then moved to the defined scan position.

[0037] For carrying out the scan process, a small liquid volume of e.g. in the magnitude of 1 μl is forced out of the pipette tip 120 such that a small bulge 130 of scanning liquid protrudes from the pipette channel and contacts the wafer surface. FIG. 3 shows a side view of the pipette tip 125 with the protruding bulge 130 of scanning liquid touching the wafer bevel 104 during this step. The wafer 100 then rotates for a little under 360°, like 355°, to avoid scanning the notch. After the scan, the scan table moves back to default position and the pipette 120 spits out the entire amount liquid contained in the pipette on the droplet holder.

[0038] Provision can be made that the volume of the bulge is adjusted during the scan to account for an assumed volume decrease of the liquid bulge due to evaporation, or an assumed volume increase of the bulge due to humidity pickup. For example, an additional volume in the magnitude of 20% of the initial bulge volume can be added every 90° of the scan. The addition or reduction of bulge volume can follow a predetermined scheme, that can be default or based on previous experimentation in a given environment.

[0039] During the scan, the bulge contour, or length of the partial droplet protruding from the pipette channel, can be monitored with the 2D camera. The pictures can be analysed to determine the success rate of the scan, for example whether there was a tear in the film connecting the bulge to the wafer surface. In terms of numbers, the length of the bulge reaching from the pipette channel to the wafer surface/bevel, in one embodiment, should preferably remain in the range of 1 mm±1 μm. Next to indicating whether the scan was successful the pictures also convey information whether the volume of the bulge changed during the scan.

[0040] In the given context of monitoring the bulge contour, provision can also be made that the monitoring is used to adjust the volume of the bulge on the basis of information extracted from the picture during the scan to account for an actual volume decrease or increase of the bulge.

[0041] After the scan, the pipette 120 transfers the liquid comprising the dissolved impurities from the droplet holder to a vial. The liquid can be diluted with either water or another scanning liquid to, for example, 650 μm (ratio 1:4), and further processed. The wafer 100 can be transferred back to an aggregate or another station. The pipette can self-clean and clean the droplet holder with a cleaning solution.