A method for automatic focusing of a patterned wafer

Abstract

The application discloses a kind of automatic focusing methods of hollowed-out wafer photoetching, belong to photoetching process technical field.The method includes: determining wafer rough focus surface;During scanning exposure process, whether scanning passes through hollowed-out area according to focusing sensor feedback value is judged;When scanning passes through wafer non-hollowed-out area, focusing Z axis adjusts exposure focus plane according to the focusing sensor feedback value and carries out exposure;When scanning passes through wafer hollowed-out area, focusing Z axis keeps the exposure focus plane on the wafer rough focus surface and does not carry out exposure.The application avoids moving to wrong exposure focus plane when scanning passes through hollowed-out area, the height difference of focus plane of lens in wafer hollowed-out area and non-hollowed-out area is reduced, and defocus phenomenon is prevented, so that the exposure effect is effectively optimized.

Description

Technical Field

[0001] This invention relates to an automatic focusing method for lithography of hollow wafers, belonging to the field of lithography technology. Background Technology

[0002] The lithography machine scans and exposes the wafer, achieving real-time focusing through a focusing system. If the wafer surface is flat, the focusing system sends a command to the Z-axis during scanning, and the Z-axis focuses and moves to the exposure focal plane in real time for exposure.

[0003] However, if there are cutouts on the wafer surface, there will be a height difference between the wafer surface and the cutouts. When the cutouts are scanned, the Z-axis is adjusted according to the focusing system commands, which may cause the lens to move to the wrong exposure focal plane. Furthermore, during the process of moving from the cutout area to the non-cutout area, the real-time focusing response of the Z-axis is slow, and the height difference between the cutout and non-cutout areas prevents the Z-axis from quickly focusing on the exposure focal plane of the non-cutout area, resulting in the exposure pattern being out of focus.

[0004] In other words, during wafer scanning exposure, some areas between the end of the cutout area in the scanning direction and the non-cutout area will experience defocusing, resulting in a poorer pattern exposure effect. Summary of the Invention

[0005] To address the defocusing issue during the scanning and exposure process of hollow wafers and optimize the exposure effect, this invention provides an automatic focusing method for hollow wafer photolithography, the technical solution of which is as follows:

[0006] Step 1: Determine the rough focal plane of the wafer;

[0007] Step 2: During the scanning exposure process, determine whether the scan has passed through the cutout area based on the feedback value from the focus sensor;

[0008] When the scan passes through the non-cutout area of ​​the wafer, the Z-axis is focused and the exposure focal plane is adjusted according to the feedback value of the focus sensor to perform exposure;

[0009] When the scan passes through the wafer cutout area, the Z-axis is focused to keep the exposure focal plane on the wafer rough focal plane without focusing exposure.

[0010] Optionally, the method for determining the rough focal plane of the wafer includes: taking n points {P} in the non-cutout area of ​​the wafer surface. n}, in {P n In the formula, each position moves the focus Z-axis to the exposure focal plane based on the feedback value from the focus sensor, and records the focus Z-axis position f of each point, resulting in a set {F} containing the focus Z-axis positions. n After removing the extreme values, the average is taken to obtain the coarse focal plane Avg of the entire wafer.

[0011] Optionally, the n points can be selected by taking n points at the flat edge of the wafer.

[0012] Optionally, the n points can be selected by finding the edge of the wafer and then selecting n points on the edge of the wafer.

[0013] Optionally, the edge finding method is as follows: take a point 3-5 mm away from the wafer center at the theoretical position of the wafer edge, move the scanning axis towards the wafer edge, and when the focus sensor feedback value determines that the scan has passed through the cutout area, take this position as the edge of the wafer.

[0014] Optionally, the scanning axis can be moved along the Y-axis towards the wafer edge.

[0015] Optionally, the n points are located at a distance of 3 to 5 mm from the flat edge or edge of the wafer.

[0016] Optionally, the focus sensor is a WDI autofocus sensor. When the feedback value is set to 0, it scans the cutout area of ​​the wafer; if the feedback value is not equal to 0, it scans the non-cutout area of ​​the wafer.

[0017] The beneficial effects of this invention are:

[0018] The automatic focusing method of the present invention, during the photolithography process of stenciling a wafer, first determines whether the focus is above the stencil area based on the feedback value of the focus sensor. When scanning to the stencil area, the focus Z-axis is kept on the coarse focal plane of the wafer to avoid moving to the wrong exposure focal plane. This reduces the height difference between the focal planes of the lens in the stencil area and the non-stencil area of ​​the wafer, prevents defocusing, and effectively optimizes the exposure effect. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of a hollowed-out wafer.

[0021] Figure 2 This is a schematic diagram showing the positions of n points selected in Embodiment 1 of the present invention.

[0022] Figure 3 This is a schematic diagram of the wafer edge obtained by edge finding in Embodiment 1 of the present invention.

[0023] Figure 4This is a schematic diagram of the scanning axis movement in the wafer edge obtained by edge finding in Embodiment 1 of the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0025] First, let's introduce the technical terms involved in this invention:

[0026] WDI is a high-speed, high-precision, autonomous digital autofocus sensor. It uses a semiconductor laser, a built-in image sensor, an FPGA (Field Programmable Gate Array), and a microprocessor for fast digital image processing.

[0027] WDI Feedback Value: WDI quickly and accurately measures the distance and direction of the focal point by projecting a laser line onto the sample and digitally processing the image; this is the feedback value. This information can then be directly output to the corresponding controller.

[0028] Flat edge: A small flat surface on the circumference of a wafer, used as a basis for crystal orientation.

[0029] Focusing Z-axis: The Z-axis is responsible for adjusting the exposure focal plane of the objective lens. As the depth of the scanning exposure focal plane changes, the Z-axis makes fine adjustments to follow the depth. The focal length of the entire working area is adjusted by the exposure focal plane of the objective lens.

[0030] Exposure focal plane: The plane where the laser passes through the focal point of the objective lens is exposed. The spot size is the smallest and the energy per unit area is the highest at the focal point.

[0031] Defocusing phenomenon: The laser passes through the plane outside the focal point of the objective lens for exposure. The greater the defocusing, the more dispersed the laser is and the larger the spot size.

[0032] Hollowed-out areas and non-hollowed-out areas: A complete wafer may have some areas drilled or grooved to create hollowed-out areas due to manufacturing processes, while the rest may be non-hollowed-out areas. Hollowed-out areas can be square, round, triangular, or other shapes.

[0033] Example 1:

[0034] This embodiment provides an automatic focusing method for lithography of cut-out wafers, the specific scheme of which includes the following:

[0035] Determine the rough focal plane of the wafer.

[0036] During wafer scanning and exposure, the focus is applied at different locations on the wafer. If there are cutout areas on the wafer, there will be a height difference between the cutout areas and the non-cutout areas.

[0037] When the scanned area is cut out, the focus sensor sends a command to the Z-axis by detecting the feedback value in real time: keep the Z-axis position at the position of the wafer coarse focal surface, without controlling the movement of the Z-axis.

[0038] When the scan passes through a non-cutout area, the real-time focusing command is sent to the Z-axis by detecting the feedback value of the focusing sensor in real time, and the Z-axis moves to the exposure focal plane position for exposure.

[0039] The focusing sensor is a WDI autofocus sensor, which detects the WDI feedback value in real time. When the WDI feedback value is 0, the wafer's cutout area is scanned, and the Z-axis is kept at the coarse focal plane without focusing or exposure. If the feedback value is not equal to 0, the non-cutout area of ​​the wafer is scanned, and normal real-time focusing and exposure are performed.

[0040] The method for determining the location of the rough focal plane on a wafer includes the following steps:

[0041] (1) Select multiple locations on the wafer;

[0042] The selected options include two types, one of which is: such as Figure 2 As shown, n points {P} are taken at a distance of 3-5 mm from the flat edge on the wafer surface. n}, which takes less time; another approach: obtain the wafer edge (including flat edges) by edge finding, and take n points {P} at a distance of 3-5mm from the wafer edge. n This allows for the acquisition of a more precise location of the coarse focal plane.

[0043] The method for finding edges is as follows: Figure 3 As shown, point 1-8 is located 3-5 mm from the theoretical position at the wafer edge towards the wafer center. At this position, the lens is moved to the exposure focal plane, and the scanning axis is moved along the Y-axis towards the wafer edge, as shown. Figure 4 As shown, when the ordinate of the selected point is greater than the ordinate of the wafer center, the scanning axis moves upward along the Y-axis; when the ordinate of the selected point is less than the ordinate of the wafer center, the scanning axis moves downward along the Y-axis. Compared to moving the scanning axis along the X-axis towards the wafer edge, this reduces the time spent finding the edge on the flat edge. When the WDI feedback value is exactly 0, this position can be taken as the edge of the wafer, and the focusing Z-axis position f can be obtained simultaneously. Figure 4 As shown, points 1`-8` represent the intersection of the scanning axis and the edge of the wafer; points 1`-5` can be fitted to the actual wafer placement position and center; points 6`-8 can be fitted to the wafer placement angle.

[0044] The advantage of this approach is that it can scan the wafer edge more accurately, obtain a relatively accurate exposure focal plane, and also obtain rotation and offset data of the actual and theoretical positions of the wafer.

[0045] (2) In {P n In the formula, each position moves the focus Z-axis to the exposure focal plane based on the WDI feedback value and records the focus Z-axis position f of each point, resulting in a set {F} containing the focus Z-axis positions. n After removing the extreme values, the average is taken to obtain a coarse focal plane Avg for the entire wafer.

[0046] After determining the coarse focal plane Avg, during the scanning stepping process, in non-cutout areas, the Z-axis is still moved to the exposure focal plane to achieve focusing based on the WDI feedback value; when scanning into the cutout area, the WDI feedback value is always 0, at which point the Z-axis is kept at the position of the coarse focal plane Avg.

[0047] In this embodiment of the autofocus method, during the photolithography process of stenciling a wafer, the focus is first determined based on the WDI feedback value to determine whether the focus is above the stencil area. When the stencil area is scanned, the focus Z-axis is kept on the coarse focal plane of the wafer to avoid moving to the wrong exposure focal plane. The height difference between the focal planes of the lens in the stencil area and the non-stencil area of ​​the wafer is reduced to prevent defocusing.

[0048] Some steps in the embodiments of the present invention can be implemented using software, and the corresponding software program can be stored in a readable storage medium, such as an optical disc or a hard disk.

[0049] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An automatic focusing method for lithography on a cutaway wafer, characterized in that, The method includes: Step 1: Determine the rough focal plane of the wafer; Step 2: During the scanning exposure process, determine whether the scan has passed through the cutout area based on the feedback value from the focus sensor; When the scan passes through the non-cutout area of ​​the wafer, the Z-axis is focused and the exposure focal plane is adjusted according to the feedback value of the focus sensor to perform exposure; When the scan passes through the wafer cutout area, the Z-axis is focused to keep the exposure focal plane on the wafer rough focal plane without focusing exposure.

2. The automatic focusing method for hollow wafer photolithography according to claim 1, characterized in that, The method for determining the rough focal plane of the wafer includes: taking n points {P} in the non-cutout area of ​​the wafer surface. n }, in {P n In the formula}, each position moves the focus Z-axis to the exposure focal plane based on the feedback value from the focus sensor, and records the focus Z-axis position f of each point, resulting in a set {F} containing the focus Z-axis positions. n After removing the extreme values, the average is taken to obtain the coarse focal plane Avg of the entire wafer.

3. The automatic focusing method for hollow wafer photolithography according to claim 2, characterized in that, The method for selecting n points is as follows: select n points at the flat edge of the wafer.

4. The automatic focusing method for hollow wafer photolithography according to claim 2, characterized in that, The method for selecting n points is as follows: obtain the edge of the wafer by finding the edge, and then select n points on the edge of the wafer.

5. The automatic focusing method for cutout wafer lithography according to claim 4, characterized in that, The edge-finding method is as follows: a point is taken 3-5 mm away from the wafer center at the theoretical position of the wafer edge, and the scanning axis is moved towards the wafer edge. When the focus sensor feedback value determines that the scan has passed through the cutout area, this position is regarded as the edge of the wafer.

6. The automatic focusing method for cut-out wafer lithography according to claim 5, characterized in that, The scanning axis moves along the Y-axis toward the edge of the wafer.

7. The automatic focusing method for cutout wafer lithography according to any one of claims 2-6, characterized in that, The n points are located at a distance of 3 to 5 mm from the flat edge or edge of the wafer.

8. The automatic focusing method for cutout wafer lithography according to any one of claims 1-7, characterized in that, The focus sensor is a WDI autofocus sensor. When the feedback value is set to 0, it scans the cutout area of ​​the wafer. If the feedback value is not equal to 0, it scans the non-cutout area of ​​the wafer.

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