Accurate measurement method for overlay of photoetching layer with inclined morphology

Abstract

The invention discloses an accurate measurement method for overlay of a photoetching layer with inclined morphology, and aims to solve the problem of inaccurate measurement caused by inclined asymmetric morphology of a thick glue overlay mark. The method comprises the following steps of: forming current-layer overlay marks with inclined asymmetric morphology on a substrate covered with thick photoresist through photoetching exposure; measuring the widths of two opposite inclined edges of the mark in at least one direction, for example, measuring the width of a first inclined edge and the width of a second inclined edge by using a critical dimension scanning electron microscope, and calculating the in-layer offset in the direction based on the width difference; measuring the original overlay error of the current-layer mark relative to the previous-layer mark by adopting overlay measurement equipment; and finally, based on the original overlay error and the calculated in-layer offset, calculating to obtain a corrected accurate overlay error. According to the method, the accuracy and the reliability of overlay measurement of the thick photoresist photoetching layer are remarkably improved by quantitatively evaluating and compensating the deviation introduced by the asymmetry of the morphology of the mark.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor technology, in particular to a precise measurement method for overlay of an inclined topography photolithography layer. Background Art

[0002] Photolithography is a key process in the manufacture of semiconductor integrated circuits. Through the photolithography process, the circuit pattern on the mask is transferred to the photoresist layer on the wafer surface. Typically, multiple photolithography and etching processes are performed on the wafer to build the complex circuit layer by layer when forming the device structure. In subsequent photolithography steps, the newly formed current layer pattern (e.g., the pattern left on the photoresist layer of the current photolithography layer) must be precisely aligned with the previous layer pattern on the wafer substrate. This alignment process is commonly known as overlay. Overlay accuracy is crucial to ensure proper connection between patterns on different layers and avoid defects such as short circuits or open circuits. Overlay error is a key indicator of photolithography alignment accuracy and directly affects device performance and yield. It is generally required that the overlay error between the current layer pattern and the previous layer pattern must be within a certain ratio (e.g., 1 / 3) of the pattern's critical dimension or minimum spacing. Ideally, the overlay error should be zero.

[0003] To monitor and control overlay accuracy, specialized overlay measurement equipment is commonly used in integrated circuit manufacturing. These devices utilize optical imaging or diffraction principles to determine overlay error by measuring the relative positional deviation between pre-set overlay marks (usually consisting of a previous layer mark and a current layer mark) on the wafer.

[0004] However, in the actual overlay measurement process, a variety of factors may introduce measurement errors, affecting the accuracy of the measurement results. These factors include, but are not limited to: the design and topography quality of the overlay mark itself (e.g., unclear marks, low contrast), process variations (e.g., mark asymmetry or topography changes introduced by the etching process or thin film deposition process), wafer deformation caused by film stress, and errors in the measurement equipment itself.

[0005] In particular, when thicker photoresist layers (e.g., up to several microns thick) are used in the photolithography process, a significant problem is that Figure 1 The overlay marks formed after photolithography exposure and development are prone to exhibit tilted and asymmetric sidewall morphology. For example, due to factors such as thicker photoresist, exposure or development conditions, the sidewalls of the overlay marks (such as BOX patterns) on the previous layer (such as BAR patterns) are not perpendicular, but have a certain tilt angle. The tilt degree or bottom width on the two sides may be inconsistent, forming an asymmetric structure. Figure 2This tilted, asymmetric topography can severely distort the optical signal collected by the overlay measurement equipment, causing the measured overlay value to deviate from the true value, thereby introducing significant systematic or random errors. Experimental data has shown that when processing thick resist layers (such as 4μm thick photoresist), the measured values ​​obtained using conventional overlay measurement methods can be abnormally large at certain fixed points in specific directions (such as the X or Y directions). This is likely caused by the asymmetric tilted topography of the mark.

[0006] This asymmetric tilted morphology of the overlay mark of the layer, which is introduced by the characteristics of the thick glue layer itself or the photolithography process, actually represents a graphic offset within the layer. Existing overlay measurement equipment based on optical principles usually has difficulty in effectively identifying and compensating for this measurement error caused by the asymmetry of the internal morphology of the mark. In actual production, when encountering such overlay measurement anomalies caused by the tilted morphology of thick glue, there is often a lack of effective automated and precise measurement methods, and sometimes it has to be downgraded to relying on manual microscopic appearance inspection to determine the alignment status. This method is inefficient and lacks precision, and is not conducive to process control and yield improvement in large-scale production.

[0007] Therefore, the industry needs a new method that can overcome the interference of the tilted and asymmetric morphology of the overlay marks caused by the thick photoresist layer on the measurement, and realize the accurate measurement of the overlay error of this type of photoresist layer to meet the strict requirements of advanced semiconductor manufacturing for overlay accuracy. Summary of the Invention

[0008] The present invention aims to solve at least the following technical problems existing in the prior art:

[0009] In semiconductor manufacturing, especially in processes involving thick photoresist layers, the layer graphics of the overlay marks formed by lithography tend to exhibit tilted and asymmetric sidewall morphology. This tilted and asymmetric morphology will seriously interfere with the signals collected by the overlay measurement equipment based on optical principles, resulting in the measured overlay error value containing deviations introduced by the asymmetry of the mark's own morphology, which cannot accurately reflect the actual inter-layer alignment situation and may result in abnormally large measurement values. Existing optical overlay measurement methods are usually unable to effectively circumvent or compensate for such measurement errors caused by mark asymmetry. In production practice, when faced with such problems, sometimes we have to resort to inefficient, low-precision and highly subjective manual appearance inspection methods to deal with them, which is not conducive to precise process control and yield management of large-scale production.

[0010] Therefore, an object of the present invention is to provide an accurate measurement method for overlay of a thick resist with an inclined topography, so as to overcome the problem of inaccurate measurement caused by the inclined asymmetric topography of the overlay mark in the prior art.

[0011] To achieve the above-mentioned and other related purposes, the present invention provides a method for accurately measuring the overlay of a tilted topography lithography layer, comprising:

[0012] Step 1: forming an overlay mark having an inclined morphology on a substrate covered with a thick photoresist by photolithographic exposure;

[0013] Step 2: measuring the widths of two relatively inclined sides of the overlay mark of the current layer in at least one direction to obtain a first inclined side width and a second inclined side width, and calculating the intra-layer offset of the overlay mark of the current layer in the at least one direction based on the first inclined side width and the second inclined side width;

[0014] Step 3: using an overlay measurement device to measure the original overlay error of the overlay mark of the current layer relative to the overlay mark of the previous layer on the substrate;

[0015] Step 4: Calculate the corrected accurate overlay error based on the original overlay error and the intra-layer offset.

[0016] Preferably, in step 1, the overlay mark of the current layer is a graphic structure suitable for overlay measurement, which is a box-frame morphology, a stripe mark morphology or a grating mark morphology.

[0017] Preferably, in step 1, the thickness of the thick photoresist is greater than 2 μm.

[0018] Preferably, in step 2, the measurement is performed using a critical dimension scanning electron microscope.

[0019] Preferably, in step 2, the first inclined edge width and the second inclined edge width of the layer overlay mark in the X direction, as well as the first inclined edge width and the second inclined edge width in the Y direction are measured; the intra-layer offset includes the intra-layer offset in the X direction and the intra-layer offset in the Y direction, and the intra-layer offset in the X direction (ΔCDX) is calculated by the following formula: ΔCDX=X-CD1-X-CD2, wherein X-CD1 represents the first inclined edge width in the X direction, and X-CD2 represents the second inclined edge width in the X direction; the intra-layer offset in the Y direction (ΔCDY) is calculated by the following formula: ΔCDY=Y-CD1-Y-CD2, wherein Y-CD1 represents the first inclined edge width in the Y direction, and Y-CD2 represents the second inclined edge width in the Y direction.

[0020] Preferably, in step 4, the precise overlay error includes the precise overlay error in the X direction and the precise overlay error in the Y direction. The precise overlay error in the X direction (OVLx) is calculated by the following formula: OVLx=OVL1x-ΔCDX, wherein OVL1x represents the component of the original overlay error in the X direction, and ΔCDX represents the intra-layer offset in the X direction; the precise overlay error in the Y direction (OVLy) is calculated by the following formula: OVLy=OVL1y-ΔCDY, wherein OVL1y represents the component of the original overlay error in the Y direction, and ΔCDY is the intra-layer offset in the Y direction.

[0021] Preferably, in step 2, the width of the inclined edge is directly measured using a slope measurement mode of the critical dimension scanning electron microscope.

[0022] Preferably, in step 2, the width of the inclined edge is calculated by measuring the line width plus the top width, and the calculation method is: inclined edge width = line width - top width; wherein, the measurement setting of the line width is: the threshold value of one side edge of the measuring line is set to 0% to measure the bottom of the line, and the threshold value of the other side edge is set to 100% to measure the top of the line.

[0023] Preferably, the measurement position in step 2 corresponds one-to-one to the overlay error measurement position in step 3.

[0024] Preferably, in step three, the overlay measurement device selects 5-10 measurement areas within the wafer, and performs measurements at four corner positions around each measurement area to obtain the original overlay error.

[0025] Preferably, the thick photoresist is i-line photoresist.

[0026] As described above, the precise measurement method of the tilted topography lithography layer overlay of the present invention has the following beneficial effects:

[0027] By quantitatively measuring the tilt asymmetry of the overlay mark in the layer (for example, using CD-SEM to measure the tilt edge width difference), the intra-layer offset introduced by the mark's inherent asymmetry is calculated. This offset is then used to correct the original error value obtained by traditional optical overlay measurement, effectively eliminating the influence of mark topography asymmetry on the measurement results and achieving a precise overlay error that is closer to the actual inter-layer alignment. This method is particularly suitable for addressing the tilt asymmetry of overlay marks caused by thick photoresist layers (e.g., greater than 2μm), overcoming the limitations of traditional optical measurement methods in such situations and avoiding measurement data anomalies and misjudgments. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1A schematic diagram showing that the overlay mark formed after photolithography exposure and development in the prior art tends to exhibit tilted and asymmetric sidewall morphology;

[0029] Figure 2 The figure shows the abnormally large actual measurement value of a fixed point in the X / Y direction in the prior art;

[0030] Figure 3 Shown is a schematic diagram of a precise measurement method for overlay of a tilted topography photolithography layer according to the present invention;

[0031] Figure 4 It is a schematic diagram showing the formation of an overlay mark with an inclined morphology according to the present invention;

[0032] Figure 5 Shown is a schematic diagram of the CDSEM measurement point positions in the present invention, taking the X direction of the layer overlay mark as an example;

[0033] Figure 6 Schematic diagram showing the calculation of the width of the inclined edge by measuring the line width plus the top width according to the present invention;

[0034] Figure 7 Shown is a schematic diagram of the line width measurement settings of the present invention. DETAILED DESCRIPTION

[0035] The following describes the embodiments of the present invention through specific examples. Those skilled in the art will readily understand the other advantages and benefits of the present invention from the disclosure herein. The present invention may also be implemented or applied through various other specific embodiments, and the details in this specification may be modified or altered based on different viewpoints and applications without departing from the spirit of the present invention.

[0036] See also Figure 3 The embodiment of the present invention provides a precise measurement method for overlay of a thick resist with inclined topography, comprising the following steps:

[0037] Step 1: Form an overlay mark with an inclined morphology on a substrate covered with thick photoresist by photolithographic exposure, forming a Figure 4 The structure shown.

[0038] This step is a standard part of the photolithography process, and its purpose is to form a mark on the current process layer for subsequent inter-layer alignment measurement. However, due to the thick photoresist (for example, it may reach 4μm or thicker in some applications), as well as the influence of process conditions such as photolithography exposure and development, the sidewalls of the current layer overlay mark formed are often not in the ideal vertical state, but show a certain tilt angle. More importantly, the degree of tilt of the sidewalls on the opposite sides may be different, resulting in the mark morphology being asymmetric in cross section (such as Figure 1The tilted asymmetric BOX mark shown).

[0039] In some embodiments, the thickness of the thick photoresist is greater than 2 μm. This thickness range is relative to conventional logic device front-end processes, and at this thickness, the tilt asymmetry of the mark is more significant.

[0040] In some embodiments, the thick photoresist is i-line photoresist. Although i-line photoresist may be more susceptible to producing such tilted asymmetric morphologies due to its characteristics, the method of the present invention is not limited thereto and is also applicable to marks with similar tilted asymmetric morphologies formed by other types of photoresists under specific conditions.

[0041] In some embodiments, when the layer overlay mark is a pattern structure suitable for overlay measurement, such as a box shape, a bar mark shape, or a grating mark shape, these mark structures are commonly used in the industry and are easy to identify and measure with optical overlay equipment. They are also suitable for subsequent morphological detail measurement using, for example, a scanning electron microscope.

[0042] Step 2: Measure the widths of two relatively inclined edges in at least one direction of the layer overlay mark to obtain a first inclined edge width and a second inclined edge width, and calculate the intra-layer offset of the layer overlay mark in at least one direction based on the first inclined edge width and the second inclined edge width.

[0043] The tilted asymmetric morphology of the mark will directly affect the signal of optical overlay measurement (such as Figure 2 The schematic signal distortion) causes the measured overlay value (OVL1) to deviate from the true inter-layer alignment value. The present invention introduces this step to quantitatively evaluate the deviation introduced by the asymmetric morphology of the mark itself, that is, the intra-layer offset (ΔCD).

[0044] In some embodiments, in step 2, a critical dimension scanning electron microscope (CD-SEM) is used for measurement. CD-SEM can provide sufficiently high resolution to accurately measure the microscopic dimensions and topographical features of the mark, including the width of the sloped sidewall. Compared to optical measurement, CD-SEM is less susceptible to interference from the mark's internal optical properties and complex topography, and the measurement results are closer to the actual geometric dimensions.

[0045] In some embodiments, see Figure 5In step 2, the first inclined edge width (X-CD1) and the second inclined edge width (X-CD2) of the layer overlay mark in the X direction, as well as the first inclined edge width (Y-CD1) and the second inclined edge width (Y-CD2) in the Y direction are measured. The intra-layer offset includes the intra-layer offset in the X direction (ΔCDX) and the intra-layer offset in the Y direction (ΔCDY). The intra-layer offset in the X direction is calculated by the following formula: ΔCDX = X-CD1-X-CD2; where X-CD1 represents the first inclined edge width in the X direction, and X-CD2 represents the second inclined edge width in the X direction. The intra-layer offset in the Y direction is calculated by the following formula: ΔCDY = Y-CD1-Y-CD2; where Y-CD1 represents the first inclined edge width in the Y direction, and Y-CD2 represents the second inclined edge width in the Y direction. The ΔCDX and ΔCDY values ​​obtained by this calculation represent the equivalent center position offset caused by the asymmetric tilted morphology of the mark in the X and Y directions.

[0046] In some embodiments, the measurement of the sloped edge width can be achieved by configuring the CD-SEM's measurement algorithm. For example, the slope measurement mode of a critical dimension scanning electron microscope can be used to directly measure the sloped edge width. Some commercial CD-SEM instruments (such as the HITACHI CDSEM) provide this specialized function for measuring sloped structural features, directly outputting the projected width of the sloped edge for ease of use.

[0047] In some embodiments, if the direct measurement mode of the inclined edge is not available or the effect is not good, a combined measurement method can also be used. Figure 6 In step 2, the width of the inclined edge is calculated by adding the line width to the top width. The calculation method is: inclined edge width = line width - top width. The line width measurement is set as follows: the threshold value of one edge of the measuring line is set to 0% to measure the bottom of the line, and the threshold value of the other edge is set to 100% to measure the top of the line (as shown in the figure). Figure 7 This method utilizes the CD-SEM's ability to identify the top and bottom edges of the line and calculates the width of the inclined portion, effectively achieving the goal.

[0048] In some embodiments, the measurement positions in step 2 correspond one-to-one with the overlay error measurement positions in step 3. This means that the CD-SEM tilt edge width measurement should be performed on the same batch of marks as those subjected to optical overlay measurement (or on representative, adjacent marks) to ensure that the calculated intra-layer offset ΔCD accurately reflects the mark topography deviation at the measurement point, thereby improving the accuracy of subsequent corrections.

[0049] Step 3: Use an overlay measurement device to measure the original overlay error (OVL1) of the overlay mark of the current layer relative to the overlay mark of the previous layer on the substrate.

[0050] This step is a routine measurement performed using standard overlay measurement equipment based on optical principles (imaging or diffraction) to obtain the original overlay measurement value OVL1 including the topographic error.

[0051] In some embodiments, in step 3, the overlay measurement equipment selects 5-10 measurement areas (shots) within the wafer and measures the four corners of each measurement area to obtain the raw overlay error (OVL1). This multi-point sampling strategy helps evaluate the distribution of overlay error across the entire wafer or within a specific area, obtaining more comprehensive raw data. Of course, the specific number and location of sampling points can be adjusted based on process requirements and equipment capabilities.

[0052] Step 4: Calculate the corrected accurate overlay error (OVL) based on the original overlay error (OVL1) and the intra-layer offset (ΔCD).

[0053] This is one of the core steps of the present invention. By subtracting the intra-layer offset ΔCD calculated by the tilted edge width measured by CD-SEM from the original overlay value OVL1 containing morphological errors obtained by optical measurement, the influence of the tilted asymmetric morphology of the mark on the measurement results is eliminated or significantly reduced.

[0054] In some embodiments, the precise overlay error includes an X-direction precise overlay error (OVLx) and a Y-direction precise overlay error (OVLy). The X-direction precise overlay error is calculated using the following formula: OVLx = OVL1x - ΔCDX; where OVL1x represents the component of the original overlay error in the X direction, and ΔCDX represents the intra-layer offset in the X direction. The Y-direction precise overlay error is calculated using the following formula: OVLy = OVL1y - ΔCDY; where OVL1y represents the component of the original overlay error in the Y direction, and ΔCDY represents the intra-layer offset in the Y direction. The calculated OVLx and OVLy are the corrected precise overlay errors that better reflect the true inter-layer alignment status.

[0055] The method proposed by the present invention can effectively solve the problem of inaccurate optical overlay measurement due to the tilted and asymmetric morphology of the overlay marks of the thick resist photolithography layer in the prior art. By introducing CD-SEM measurement to quantitatively evaluate the intra-layer offset of the mark itself, and using this offset to correct the optical measurement results, accurate overlay error is finally obtained. This not only improves the accuracy and reliability of overlay measurement, avoiding process misjudgments or unnecessary rework due to measurement errors, but also provides a quantitative and automatable measurement process, replacing the inefficient and highly subjective manual appearance inspection method. Ultimately, it helps to improve the yield and production efficiency of semiconductor products using thick resist photolithography technology.

[0056] It should be noted that the order of the method steps described in the present invention is only an example. In actual applications, the order of step 2 (CD measurement) and step 3 (OVL measurement) can be adjusted according to the production process and equipment arrangement. For example, OVL1 measurement can be performed first, and then CD measurement, as long as these two data can be obtained for the final correction calculation.

[0057] In addition, although the present invention uses CD-SEM as the preferred tool for measuring the width of the inclined edge, in theory, other technologies that can accurately measure the size of the inclined sidewall of the microstructure may also be used to perform the measurement in step 2 under applicable conditions.

[0058] It should be noted that the illustrations provided in this embodiment are only used to schematically illustrate the basic concept of the present invention. Therefore, the illustrations only show components related to the present invention and are not drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component can be changed at will, and the component layout type may also be more complicated.

[0059] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Anyone skilled in the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by one of ordinary skill in the art without departing from the spirit and technical principles disclosed herein are intended to be covered by the claims of the present invention.

Claims

1. A precise measurement method for overlay of tilted topography lithography layers, characterized in that: At least: Step 1: forming an overlay mark having an inclined morphology on a substrate covered with a thick photoresist by photolithographic exposure; Step 2: measuring the widths of two relatively inclined sides of the overlay mark of the current layer in at least one direction to obtain a first inclined side width and a second inclined side width, and calculating the intra-layer offset of the overlay mark of the current layer in the at least one direction based on the first inclined side width and the second inclined side width; Step 3: using an overlay measurement device to measure the original overlay error of the overlay mark of the current layer relative to the overlay mark of the previous layer on the substrate; Step 4: Calculate the corrected accurate overlay error based on the original overlay error and the intra-layer offset.

2. The precise measurement method for overlay of tilted topography lithography layers according to claim 1, characterized in that: In step 1, the overlay mark of the current layer is a graphic structure suitable for overlay measurement, which is a box-frame morphology, a stripe mark morphology or a grating mark morphology.

3. The precise measurement method for overlay of tilted topography lithography layers according to claim 1, characterized in that: In step 1, the thickness of the thick photoresist is greater than 2 μm.

4. The precise measurement method for overlay of tilted topography lithography layers according to claim 1, characterized in that: In step 2, the measurement is performed using a critical dimension scanning electron microscope.

5. The precise measurement method for overlay of tilted topography lithography layer according to claim 1, characterized in that: In step 2, the first inclined edge width and the second inclined edge width of the layer overlay mark in the X direction, as well as the first inclined edge width and the second inclined edge width in the Y direction, are measured; the intra-layer offset includes the intra-layer offset in the X direction and the intra-layer offset in the Y direction, and the intra-layer offset in the X direction (ΔCDX) is calculated by the following formula: ΔCDX=X-CD1-X-CD2, wherein X-CD1 represents the first inclined edge width in the X direction, and X-CD2 represents the second inclined edge width in the X direction; the intra-layer offset in the Y direction (ΔCDY) is calculated by the following formula: ΔCDY=Y-CD1-Y-CD2, wherein Y-CD1 represents the first inclined edge width in the Y direction, and Y-CD2 represents the second inclined edge width in the Y direction.

6. The precise measurement method for overlay of an inclined topography layer according to claim 5, characterized in that: In step 4, the precise overlay error includes the precise overlay error in the X direction and the precise overlay error in the Y direction. The precise overlay error in the X direction (OVLx) is calculated by the following formula: OVLx=OVL1x-ΔCDX, where OVL1x represents the component of the original overlay error in the X direction, and ΔCDX represents the intra-layer offset in the X direction; the precise overlay error in the Y direction (OVLy) is calculated by the following formula: OVLy=OVL1y-ΔCDY, where OVL1y represents the component of the original overlay error in the Y direction, and ΔCDY is the intra-layer offset in the Y direction.

7. The precise measurement method for overlay of tilted topography lithography layer according to claim 4, characterized in that: In step 2, the slope measurement mode of the critical dimension scanning electron microscope is used to directly measure the width of the inclined edge.

8. The precise measurement method for overlay of tilted topography lithography layer according to claim 1, characterized in that: In step 2, the width of the inclined edge is calculated by measuring the line width plus the top width, and the calculation method is: inclined edge width = line width - top width; wherein, the measurement setting of the line width is: the threshold value of one side edge of the measuring line is set to 0% to measure the bottom of the line, and the threshold value of the other side edge is set to 100% to measure the top of the line.

9. The precise measurement method for overlay of tilted topography lithography layer according to claim 1, characterized in that: The measurement positions in step 2 correspond one-to-one to the overlay error measurement positions in step 3.

10. The precise measurement method for overlay of tilted topography lithography layer according to claim 1, characterized in that: In step three, the overlay measurement device selects 5-10 measurement areas within the wafer, and performs measurements at four corner positions around each measurement area to obtain the original overlay error.

11. The precise measurement method for overlay of tilted topography lithography layer according to claim 3, characterized in that: The thick photoresist is i-line photoresist.

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