A method for improving overlay accuracy
By setting overlay accuracy markers on the photomask layer and calculating pre-compensation amounts, the overlay adjustment values were optimized, solving the overlay accuracy error problem caused by the offset between the photomask stage and the wafer stage, as well as wafer expansion, thus improving the overlay accuracy of the exposure machine and the product yield.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- SOI MICRO CO LTD
- Filing Date
- 2022-10-27
- Publication Date
- 2026-06-26
AI Technical Summary
In the existing technology, the photomask stage and wafer stage of the exposure machine will be offset during operation. Machine vibration causes deviation, and the wafer expansion coefficient causes deformation, resulting in large overlay accuracy errors and affecting product yield.
Set overlay accuracy marks in each alignment mark area of the photomask layer, calculate the overlay adjustment amount through the wafer pre-alignment system, and calculate the pre-compensation amount based on the measurement parameters to eliminate miscompensation and optimize the overlay adjustment value.
It improves overlay accuracy, reduces miscompensation caused by exposure, avoids errors caused by deformation of adjacent layers, optimizes the accuracy of the compensation system, and improves product yield.
Smart Images
Figure CN115576173B_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of semiconductor technology, and in particular to a method for improving the overlay accuracy during wafer exposure. Background Technology
[0002] When an exposure machine exposes a silicon wafer, it needs to use a pre-alignment system to coordinate and adjust the photomask layer, the photomask stage, and the wafer stage to transfer the photomask pattern onto the wafer.
[0003] In related technologies, the photomask stage and wafer stage of the exposure machine inevitably experience offsets during operation, and machine vibrations also cause deviations. Furthermore, the wafer itself will deform after exposure due to the coefficient of thermal expansion of different regions. However, this deformation on the wafer is a change in the overall material structure, not an overlay deviation caused by the equipment. The pre-alignment system mistakenly interprets this deformation as a difference in exposure between layers caused by the equipment and includes it in the model calculations of the pre-alignment system. This can result in excessive cumulative errors on the wafer stage, causing the overall exposure to shift to one side, affecting the overlay accuracy of the wafer and the product yield. Summary of the Invention
[0004] This application provides a method for improving overlay accuracy, solving the problem of low precision error in wafer overlay in related technologies. The method includes:
[0005] Set overlay accuracy marks in each alignment mark area of the photomask layer;
[0006] The overlay adjustment amount corresponding to the photomask layer is calculated by the wafer pre-alignment system. The overlay adjustment amount is calculated based on the measurement parameters of wafer exposure, including the symmetry and asymmetry rotation parameters and expansion parameters of the exposure unit in each direction. The overlay adjustment amount includes the error compensation calculated by the machine due to the deformation of the exposure area of adjacent layers. This error compensation will cause the wafer exposure direction offset error to exceed the error threshold.
[0007] The pre-compensation amount of the photomask layer is calculated based on the set overlay accuracy mark and the measurement parameters of wafer exposure; the pre-compensation amount is used to eliminate the miscompensation in the overlay adjustment amount;
[0008] The overlay adjustment value for the next wafer exposure process is calculated based on the overlay adjustment amount and the pre-compensation amount.
[0009] Furthermore, setting overlay accuracy marks in each alignment mark area of the photomask layer includes:
[0010] Obtain the photomask layer during the current exposure process, wherein the exposure layer is provided with a preset number of alignment mark areas;
[0011] The overlay accuracy mark is set in the alignment mark area, and the overlay accuracy mark does not coincide with the alignment identification mark in the alignment mark area. Each alignment mark area contains four alignment identification marks for locating the position of the corresponding alignment mark area.
[0012] The exposure machine exposes the wafer on the wafer stage according to the wafer pre-alignment system and transfers the photomask pattern onto the wafer.
[0013] Further, the step of calculating the overlay adjustment amount corresponding to the photomask layer through the wafer pre-alignment system includes:
[0014] Obtain the actual overlay results on the wafer surface, and measure the offset, symmetric expansion and asymmetric expansion of each alignment mark area in the x-axis direction, and the offset, symmetric expansion and asymmetric expansion in the y-axis direction during the exposure process;
[0015] Based on the measurement parameters and the wafer pre-alignment system, a model prediction is performed to obtain the overlay adjustment amount for the machine in the next exposure process.
[0016] Further, the calculation of the pre-compensation amount of the photomask layer based on the set overlay accuracy mark and the wafer exposure measurement parameters includes:
[0017] The wafer pre-alignment system calculates the predicted compensation amount of each alignment mark region in the x-axis and y-axis directions; and calculates the correction compensation amount of the corresponding alignment mark region in the x-axis and y-axis directions based on the overlay accuracy mark.
[0018] The pre-compensation amount in the x-axis direction is calculated based on the difference between the predicted compensation amount and the miscompensation amount in the x-axis direction, and the pre-compensation amount in the y-axis direction is calculated based on the difference between the predicted compensation amount and the correction compensation amount in the y-axis direction.
[0019] Further, the step of calculating the overlay adjustment value for the next wafer exposure process based on the overlay adjustment amount and the pre-compensation amount includes:
[0020] Based on the difference between the overlay adjustment amount and the pre-compensation amount in the x-axis direction, the overlay adjustment value of the exposure machine in the next exposure process is calculated, and the overlay adjustment value eliminates the miscompensation calculated by the wafer pre-alignment system in the previous layer exposure.
[0021] Furthermore, the wafer pre-alignment system is established based on a pre-alignment model. By measuring the measurement parameters after wafer exposure and performing model calculations, the overlay adjustment amount is predicted.
[0022] Furthermore, the overlay adjustment value also includes the process error of the exposure machine. The exposure machine adjusts the photomask layer, photomask stage, and wafer stage by means of the process error and the overlay adjustment value, thereby improving the overlay accuracy of the exposure to the accuracy threshold.
[0023] The beneficial effects of the above technical solution include at least the following: Based on the original wafer pre-alignment system, this application optimizes the overlay adjustment amount output by the original wafer pre-compensation system, reduces the miscompensation caused by the difference between the front and back layers due to exposure, improves the overlay accuracy, and effectively avoids miscompensation caused by the deformation of the exposure area of adjacent layers. Combined with the adjustment of mechanical overlay deviation in the pre-compensation system, the optimized compensation system can output more accurate overlay adjustment values. Attached Figure Description
[0024] Figure 1 This is a schematic diagram of the structure of the exposure machine alignment mechanism provided in the embodiments of this application;
[0025] Figure 2 This is a flowchart of a method for improving overlay accuracy provided in an embodiment of this application;
[0026] Figure 3 This is a schematic diagram of the alignment mark area provided in an embodiment of this application. Detailed Implementation
[0027] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application will be described in further detail below with reference to the accompanying drawings.
[0028] In this article, "multiple" refers to two or more. "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.
[0029] like Figure 1 As shown, in related technologies, when using a wafer pre-alignment system for photomask alignment, a pre-exposure is performed based on the photomask layer and alignment marks to transfer the photomask pattern onto the wafer. Subsequently, a measurement system calculates the actual deviation of the alignment marks or preset mark areas. The parameters measured by the pre-alignment system include the offset Tx of the measurement point in the X-axis direction, the expansion Exp_x in the X-axis direction, the offset Ty in the Y-axis direction, the expansion Exp_y in the Y-axis direction, the wafer rotation ROT, the wafer orthogonality (Non-ortho), and the wafer orthogonality (rotational asymmetry in the X and Y directions), etc.
[0030] Figure 1This is a schematic diagram of the alignment mechanism of the exposure machine. Taking the x-axis as an example, after the exposure machine performs photomask alignment and exposure based on the wafer pre-alignment system, in Lot1, due to the wafer's expansion coefficient, the marker point or marker area expands in the x-direction. At this time, the measured and calculated expansion of the X-symmetry of the exposure unit is R_Mag = 0.5. Therefore, the actual alignment mark is offset to the left by -5nm compared to the ideal alignment mark (this offset is not actually caused by machine alignment). In Lot2, the marker point or marker area shrinks in the x-direction. At this time, the expansion of the X-symmetry of the exposure unit is R_Mag = -0.2. Therefore, the actual alignment mark is offset to the right by +2nm compared to the ideal alignment mark. During both alignment processes, the exposure machine uses the calculated offset as the basis for machine action adjustment, such as moving the wafer stage and / or photomask stage to further align the alignment mark. The two shot levels cause a cumulative alignment deviation of 7nm. The more exposures, the greater the cumulative error, resulting in a larger wafer overlay deviation.
[0031] Figure 2 This is a flowchart of a method for improving overlay accuracy provided in an embodiment of this application, including the following steps:
[0032] Step 201: Obtain the photomask layer in the current exposure process. The exposure layer contains a preset number of alignment mark areas.
[0033] Because the pre-alignment system incorporates the symmetrical and asymmetrical expansion of the wafer within a specific exposure area into the error adjustment of the next exposure stage, it needs to be improved to eliminate this error. In this solution, one or more alignment mark areas are set in the photomask layer of the exposed wafer. In one possible implementation, the alignment mark areas are set as regions uniformly distributed around the circumference, and one or more concentric circles of alignment mark areas can be set. Figure 3 As shown, two concentric alignment marker areas are set on the photomask layer, surrounding the geometric center of the photomask layer. In this scheme, various offset values are calculated based on coordinate axes established at the center or at the edge positions.
[0034] Step 202: Set an overlay accuracy mark in the alignment mark area, and the overlay accuracy mark does not coincide with the alignment identification mark in the alignment mark area.
[0035] like Figure 3 As shown, positioning marks are set in each alignment mark area. The alignment mark area is set as a rectangle, and the positioning marks are located at the four corners of the rectangular alignment mark area for identification and measurement by the measuring instrument. The overlay accuracy mark is located in the area outside the four alignment identification marks within the same area, and they do not overlap. Optionally, the positioning marks and alignment identification marks represent one step accuracy of the exposure machine.
[0036] Step 203: Obtain the actual overlay results on the wafer surface, and measure the offset, symmetric expansion and asymmetric expansion of each alignment mark area in the x-axis direction, and the offset, symmetric expansion and asymmetric expansion in the y-axis direction during the exposure process.
[0037] After the exposure machine performs one exposure overlay according to the photomask layer, the photomask pattern is transferred to the wafer. Subsequently, the actual overlay result on the wafer surface is detected by a metrology system. The measurements include the offset Tx, symmetric expansion R_Mag (x-axis direction), and asymmetric expansion AR_Mag (x-axis direction) of each alignment mark area during the exposure process, as well as the offset Tx, symmetric expansion R_Mag (y-axis direction), and asymmetric expansion AR_Mag (y-axis direction) of each alignment mark area. It should be noted that the data measured by the wafer pre-alignment system also includes rotation and orthogonality in various directions. When optimizing this scheme, it is necessary to capture the symmetric and asymmetric expansion data.
[0038] Step 204: Based on the measurement parameters and the wafer pre-alignment system, perform model prediction to obtain the overlay adjustment amount for the machine in the next exposure process.
[0039] The wafer pre-alignment system is based on a pre-alignment model. Measured data is input into the model to predict the overall and partial variation trends of the wafer, ultimately outputting an overlay adjustment amount (Pt). This overlay adjustment amount is used for adjustments in the next exposure cycle, and the adjustments include at least the photomask, photomask stage, and wafer stage. The overlay adjustment amount (Pt) includes adjustments for predicted overlay deviations caused by mechanical factors, as well as compensation for errors in calculations due to deformation of adjacent layer exposure areas.
[0040] Step 205: Calculate the predicted compensation amount of each alignment mark area in the x-axis and y-axis directions using the wafer pre-alignment system; calculate the correction compensation amount of the corresponding alignment mark area in the x-axis and y-axis directions based on the overlay accuracy mark.
[0041] As mentioned earlier, the overlay adjustment amount output by the wafer pre-alignment system includes miscompensation calculated due to deformation of adjacent layer exposure areas by the machine stage. This miscompensation is not machine-induced, and if it is included in the overlay adjustment amount by default, it will cause the wafer exposure direction offset error to exceed the error threshold. In this solution, after calculating the overlay adjustment amount, the predicted compensation amounts T_x(-1,model) and T_y(-1,model) in the x-axis and y-axis directions of each alignment mark area are obtained sequentially from the wafer pre-alignment system. Based on the overlay accuracy mark, the corrected compensation amounts shift_x(-1) and shift_y(-1) in the x-axis and y-axis directions of the corresponding alignment mark area are calculated. The -1 represents the relevant data in the previous layer exposure process. The corrected compensation amount is the wafer offset in the x-axis direction calculated based on the overlay accuracy mark, such as the distance of the mark point from the center or edge area (x-axis direction or y-axis direction), while the predicted compensation amount is predicted based on the distance of the entire alignment mark area or exposure area from the center or edge area.
[0042] Step 206: Calculate the pre-compensation amount in the x-axis direction based on the difference between the predicted compensation amount and the corrected compensation amount in the x-axis direction, and calculate the pre-compensation amount in the y-axis direction based on the difference between the predicted compensation amount and the corrected compensation amount in the y-axis direction.
[0043] The formula can be expressed as: T_x(-1,intra)=shift_x(-1)-T_x(-1,model)
[0044] T_y(-1,intra)=shift_y(-1)-T_y(-1,model)
[0045] T_x(-1,intra) represents the pre-compensation amount in the x-axis direction of the previous layer, and T_y(-1,intra) represents the pre-compensation amount in the y-axis direction of the previous layer. This is the change in the overlay accuracy markings in the alignment mark area after wafer deformation (miscompensation). It should be noted that in this scheme, overlay deviations caused by other factors are included in the overlay adjustment amount, such as overlay deviations caused by mechanical structures.
[0046] Step 207: Based on the difference between the overlay adjustment amount and the pre-compensation amount in the x-axis direction, calculate the overlay adjustment value of the exposure machine in the next exposure process.
[0047] It should be noted that other process deviations may occur during the exposure process. These deviations are not the subject of the wafer exposure expansion description in this solution. The process deviation is PIE. Assuming that the adjustment value for the lower layer overlay is PC, the formula can be expressed as: PC=PIE+Pt-T_x(-1,intra)-T_y(-1,intra).
[0048] It should be noted that this solution includes multiple alignment mark areas and overlay accuracy marks. The predictive compensation system will perform comprehensive calculations based on all measured values. Since the expansion degree of each region of the wafer is different, multiple sets of values can be calculated to represent the pre-compensation amount of different alignment mark areas. Then, weighted calculations or average calculations are performed to obtain the final overlay adjustment value.
[0049] In summary, this application optimizes the existing wafer pre-alignment system by reducing the overlay adjustment amount output by the existing wafer pre-compensation system, thus improving the overlay accuracy and effectively avoiding the overlay error caused by the deformation of the exposure area of adjacent layers. Combined with the adjustment of mechanical overlay deviation in the pre-compensation system, the optimized compensation system can output more accurate overlay adjustment values.
[0050] The preferred embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above. The devices and structures not described in detail should be understood as being implemented in a conventional manner in the art. Any person skilled in the art can make many possible changes and modifications, or equivalent changes to equivalent embodiments without departing from the technical solution of the present invention. This does not affect the substantive content of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall still fall within the protection scope of the technical solution of the present invention.
Claims
1. A method for improving overlay accuracy, characterized in that, The method includes: Set overlay accuracy marks in each alignment mark area of the photomask layer; The overlay adjustment amount corresponding to the photomask layer is calculated by the wafer pre-alignment system. The overlay adjustment amount is calculated based on the measurement parameters of wafer exposure, including the symmetry and asymmetry rotation parameters and expansion parameters of the exposure unit in each direction. The overlay adjustment amount includes the error compensation calculated by the machine due to the deformation of the exposure area of adjacent layers. This error compensation will cause the wafer exposure direction offset error to exceed the error threshold. The pre-compensation amount of the photomask layer is calculated based on the set overlay accuracy mark and the wafer exposure measurement parameters; the pre-compensation amount is used to eliminate the miscompensation in the overlay adjustment amount; specifically, the predicted compensation amount of each alignment mark area in the x-axis direction and y-axis direction is calculated through the wafer pre-alignment system; the correction compensation amount of the corresponding alignment mark area in the x-axis direction and y-axis direction is calculated based on the overlay accuracy mark; The pre-compensation amount in the x-axis direction is calculated based on the difference between the predicted compensation amount and the corrected compensation amount in the x-axis direction, and the pre-compensation amount in the y-axis direction is calculated based on the difference between the predicted compensation amount and the corrected compensation amount in the y-axis direction. The overlay adjustment value for the next wafer exposure process is calculated based on the overlay adjustment amount and the pre-compensation amount.
2. The method according to claim 1, characterized in that, Setting overlay accuracy marks in each alignment mark area of the photomask layer includes: Obtain the photomask layer during the current exposure process, wherein the exposure layer is provided with a preset number of alignment mark areas; The overlay accuracy mark is set in the alignment mark area, and the overlay accuracy mark does not coincide with the alignment identification mark in the alignment mark area. Each alignment mark area contains four alignment identification marks for locating the position of the corresponding alignment mark area. The exposure machine exposes the wafer on the wafer stage according to the wafer pre-alignment system and transfers the photomask pattern onto the wafer.
3. The method according to claim 2, characterized in that, The calculation of the overlay adjustment amount corresponding to the photomask layer through the wafer pre-alignment system includes: Obtain the actual overlay results on the wafer surface, and measure the offset, symmetric expansion and asymmetric expansion of each alignment mark area in the x-axis direction, and the offset, symmetric expansion and asymmetric expansion in the y-axis direction during the exposure process; Based on the measurement parameters and the wafer pre-alignment system, a model prediction is performed to obtain the overlay adjustment amount for the machine in the next exposure process.
4. The method according to claim 3, characterized in that, The calculation of the overlay adjustment value for the next wafer exposure process based on the overlay adjustment amount and the pre-compensation amount includes: Based on the difference between the overlay adjustment amount and the pre-compensation amount in the x-axis direction, the overlay adjustment value of the exposure machine in the next exposure process is calculated, and the overlay adjustment value eliminates the miscompensation calculated by the wafer pre-alignment system in the previous layer exposure.
5. The method according to any one of claims 1 to 4, characterized in that, The wafer pre-alignment system is based on a pre-alignment model. By measuring the measurement parameters after wafer exposure and performing model calculations, the overlay adjustment amount is predicted.
6. The method according to claim 5, characterized in that, The overlay adjustment value also includes the process error of the exposure machine. The exposure machine adjusts the photomask layer, photomask stage and wafer stage by the process error and the overlay adjustment value to improve the overlay accuracy of the exposure to the accuracy threshold.