Producing milled structural elements with flat upper surfaces

Abstract

A milling machine, a non-transitory computer readable medium, and a method for milling a multi-layer object. The method may include: milling each structural element in an array of structural elements separated from each other by gaps to provide a milled structural element, wherein each milled structural element has a flat upper surface, wherein, before milling, each structural element in the array has a flat upper surface of a certain width, wherein the certain width is nanometer-scale. Milling each structural element of the array may include scanning a defocused ion beam of a certain width along a longitudinal axis of the structural element. The current intensity of the defocused ion beam decreases with distance from the middle of the defocused ion beam.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of priority to U.S. Non-Provisional Application No. 16 / 530,331, filed on August 2, 2019, which is hereby incorporated by reference in its entirety for all purposes. Background Art

[0003] Objects such as various semiconductor objects (e.g., semiconductor wafers, display panels, solar wafers) may include an array of structural elements having a substantially rectangular cross-section and separated from each other by empty gaps. Non-limiting examples of such structural elements may include memory cells, memory arrays, etc.

[0004] Uniform milling of these structural elements will result in the milled structural elements having an inclined cross-section, such as a triangular cross-section.Uniform milling is obtained when substantially every point receives substantially the same amount of radiation during the milling process.

[0005] A triangular cross section is obtained because the number of emitted material atoms increases near the edge of the structural element. The chance of irradiated material atoms escaping from the structural element increases as the irradiated material atoms are closer to any outer surface of the structural element. Thus, irradiated material atoms closer to the sidewalls of the structural element may escape through the sidewalls of the structural element - and not just through the upper surface of the structural element. Thus, the actual milling yield increases as a function of the distance from the center of the structural element.

[0006] Figure 1 Depicting from top to bottom:

[0007] a. Array of structural elements 111 before milling. The structural elements 111 are spaced apart from each other by gaps 115. Each structural element has a flat top surface 112 and vertical side walls 113 and 114. It is assumed that the different structural elements of the array are ideally identical.

[0008] b. An array of partially milled structural elements 121. The partially milled structural elements 121 are the result of a first milling iteration. The top of each of the partially milled structural elements has a triangular cross-section. The lower portion of each of the partially milled structural elements has side walls. The partially milled structural elements 121 are separated by gaps 125.

[0009] c. An array of milled structural elements 131 . The milled structural elements are the result of the second milling iteration. Each of the milled structural elements 131 has a triangular cross-section. The milled structural elements 131 are separated by gaps 135 .

[0010] There is an increasing need to produce milled structural elements with a flat top surface. Summary of the invention

[0011] A method for producing a milled structural element may be provided, the method comprising: milling each structural element in an array of structural elements separated from each other by gaps to provide a milled structural element, wherein each milled structural element of the array has a flat upper surface; wherein before milling, each structural element has a flat upper surface of a certain width, wherein the certain width is nanometer-scale. The method also includes: wherein the milling includes scanning a defocused ion beam of a certain width along the longitudinal axis of each structural element. The method also includes: wherein the current intensity of the defocused ion beam decreases with distance from the middle of the defocused ion beam.

[0012] A milling machine can be provided, comprising: a controller. The milling machine further comprises a focused ion beam module. The milling machine further comprises wherein the focused ion beam module is configured to, under the control of the controller, mill each structural element in an array of structural elements separated from each other by gaps to provide milled structural elements, wherein each milled structural element has a flat upper surface; wherein before milling, each structural element has a flat upper surface of a certain width, wherein the certain width is nanometer-level. The milling machine also comprises: wherein the milling of each structural element by the focused ion beam module comprises: scanning a defocused ion beam of a certain width along the longitudinal axis of each structural element. The milling machine also comprises: wherein the current intensity of the defocused ion beam decreases with the distance from the middle of the defocused ion beam.

[0013] A non-transitory computer readable medium may be provided, the non-transitory computer readable medium storing instructions for producing a milled structural element, the instructions causing a milling machine to perform a method comprising: milling each structural element in an array of structural elements separated from each other by gaps to provide a milled structural element, each milled structural element having a flat upper surface; wherein, prior to milling, each structural element has a flat upper surface of a certain width, wherein the certain width is nanometer-scale. The non-transitory computer readable medium further comprises wherein the milling step comprises scanning a defocused ion beam of a certain width along a longitudinal axis of each structural element. The non-transitory computer readable medium further comprises wherein the current intensity of the defocused ion beam decreases with distance from the middle of the defocused ion beam. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Examples of the claimed subject matter may be particularly pointed out and distinctly claimed in the concluding portion of the specification. However, embodiments of the disclosure, both as to organization and method of operation, together with objects, features, and advantages thereof, may be more fully understood by reference to the following detailed description in conjunction with the accompanying drawings. Figure 1 is best understood when read together, where:

[0015] Figure 1 are examples of prior art structural elements, prior art partially milled structural elements, and prior art milled structural elements;

[0016] Figure 2 According to one or more embodiments of the present disclosure are examples of structural elements, milled structural elements, and defocused ion beam current intensity distributions;

[0017] Figure 3 An example of a method is shown;

[0018] Figure 4 One or more embodiments according to the present disclosure are examples of structural elements, scan lines of a scan pattern, and a defocused ion beam current intensity distribution;

[0019] Figure 5 An example of a portion of a structural element and a scanning pattern according to one or more embodiments of the present disclosure; and

[0020] Figure 6 is an example of a focused ion beam module and controller.

[0021] It should be understood that for simplicity and clarity of description, the elements shown in the drawings are not necessarily drawn to scale. For example, for greater clarity, the size of some elements may be enlarged relative to other elements. In addition, reference numerals may be repeated in the drawings to indicate corresponding or similar elements when deemed appropriate. DETAILED DESCRIPTION

[0022] In the following specific embodiments, many specific details are explained in order to provide a thorough understanding of the embodiments of the present disclosure. However, those skilled in the art will appreciate that the current embodiments of the present disclosure can be practiced without these specific details. In other examples, well-known methods, procedures and elements are not described in detail in order not to confuse the current embodiments of the present disclosure.

[0023] The subject matter which is regarded as an embodiment of the present disclosure is particularly pointed out and distinctly claimed in the concluding portion of the specification. However, embodiments of the present disclosure, both as to organization and method of operation, together with objects, features, and advantages thereof, may be further understood by reference to the following detailed description in conjunction with the accompanying drawings. Figure 1 are best understood when read together.

[0024] Because the illustrated embodiments of the current embodiments of the present disclosure can, in most cases, be implemented using electronic components and circuits well known to those skilled in the art, the details will not be explained in more extended detail than is considered necessary as described above in order to understand and appreciate the basic concepts of the current embodiments of the present disclosure and in order not to obscure or distract from the teachings of the current embodiments of the present disclosure.

[0025] Any reference in the specification to a method shall, where necessary, apply mutatis mutandis to a milling machine capable of carrying out the method and, in detail, to a non-transitory computer-readable medium storing instructions which, once executed by a computer, result in the performance of the method.

[0026] Any reference in the specification to a system shall apply mutatis mutandis to methods that may be performed by the system and shall apply mutatis mutandis to a non-transitory computer readable medium storing instructions executable by a milling machine.

[0027] Any reference in the specification to a non-transitory computer readable medium shall apply mutatis mutandis to a milling machine capable of executing instructions stored in a non-transitory computer readable medium and shall apply mutatis mutandis to a method that may be performed by a computer reading the instructions stored in a non-transitory computer readable medium.

[0028] A milling machine, method, and non-transitory computer readable medium for milling structural elements that are spaced apart from each other by a gap may be provided to provide a milled structural element having a flat top surface.

[0029] There is a need to compensate for differences in actual milling yields across the structural element and provide irradiated atomic material with substantially the same opportunity to exit the structural element regardless of the distance between the irradiated material atoms and the sidewalls of the structural element.

[0030] The term "substantially" may refer to a limited amount of deviation. For example - at most 5%, 10%, 15%, etc.

[0031] Figure 2 1 is an example of a structural element 111 , a milled structural element 151 , and a defocused ion beam current intensity distribution 162 according to one or more embodiments of the present disclosure.

[0032] Figure 2 Depicting from top to bottom:

[0033] a. Example of a defocused ion beam current intensity distribution 162. The current intensity of the defocused ion beam decreases with distance from the middle 163 of the defocused ion beam. The current intensity is higher in the middle 163 of the defocused ion beam and lower near the edge of the defocused ion beam. This provides substantially the same opportunity for the irradiated atomic material to leave the structural element regardless of the distance between the irradiated material atoms and the sidewalls of the structural element.

[0034] b. An array of structural elements 111 before milling. The structural elements 111 are spaced apart from each other by gaps 115, have a flat top surface 112 and vertical sidewalls 113 and 114. The gaps 115 may be spaces between the vertical sidewalls of the structural elements 111. The gaps may be air gaps created during the semiconductor manufacturing process and will be filled with material in subsequent steps of the semiconductor manufacturing process. The height of each structural element is indicated as 116. The width of each structural element is indicated as 117.

[0035] c. An array of milled structural elements 151 having a rectangular cross section. Each milled structural element 151 has a flat top surface 152 and vertical side walls 153 and 154. The milled structural elements 151 are separated by gaps 155. The height of each milled structural element is indicated as 118.

[0036] Figure 3 An example of a method 200 for producing a milled structural element is shown.

[0037] Method 200 may begin with step 210 of receiving structural element information related to an array of structural elements. The array includes structural elements separated from each other by gaps. Each structural element may have a flat upper surface of a certain width. The certain width is nanometer-scale (e.g., between 100 nanometers and 1000 nanometers). Each structural element may have a vertical sidewall.

[0038] The structure element information may define a specific width (of each structure element).

[0039] Step 210 may be followed by step 220 of determining a milling plan based at least in part on the structural element information.

[0040] Step 220 may include at least one of the following:

[0041] a. Step 221, determining at least one milling parameter to provide a defocused ion beam with a certain width. The defocused ion beam has a certain width at a position where the defocused ion beam intersects the flat upper surface of the structural element.

[0042] b. Step 222, determine the desired current intensity distribution of the defocused ion beam.

[0043] c. Step 223, determining a milling scan pattern and one or more additional milling parameters. The one or more additional milling parameters may include a defocus intensity of the defocused ion beam and a defocused ion beam current distribution.

[0044] d. Step 224, receiving or determining the shape of the upper portion of the uniformly milled structural elements of the array.

[0045] e. Step 225, determining a milling plan to compensate for the deviation in shape of the upper portion of the uniformly milled structural element (relative to the flat top surface). Step 225 may be preceded by step 224. For example, assuming that the first function describes the cross-section of the top portion of the uniformly milled structural element, the milling plan should apply to the structural element a radiation that follows a second function that is substantially equal to the first function.

[0046] f. Step 226 , determining a milling plan based at least on at least one of a plurality of materials (of the structural element) to be milled during the milling process.

[0047] g. Step 227, determining a milling plan based at least on the size of the structural element to be milled.

[0048] h. Step 228, determining a milling plan based at least on the smoothness of the top surface of the structural element being milled.

[0049] i. Step 229, determine the milling rate.

[0050] The milling parameters determined during step 220 may include at least one of the following: defocus intensity, duration of milling, bias voltage provided to the objective lens, total ion beam energy, total ion beam current.

[0051] The defocus intensity can affect the current distribution of the defocused ion beam.

[0052] The duration of the milling process can determine the reduction in height of the component caused by the milling process.

[0053] The objective lens bias voltage controls the defocus intensity.

[0054] The total defocused ion beam energy does not affect the defocusing, but a stronger defocused ion beam energy allows the defocused ion beam to increase the height reduction of the structural element and can reduce the top surface deformation.

[0055] The total ion beam current does not determine defocusing, but determines the milling speed. For example, a higher ion beam current results in faster milling.

[0056] Step 220 may include using a mapping between the milling scheme and one or more parameters of the milled and / or non-milled structural elements. The mapping may be learned in one or more ways. For example, the mapping may be based on actual milling operations performed when different milling parameters are applied. Only a portion of all possible combinations of milling parameter values ​​may be tested. As another example, the milling operation may be based on a simulation or other estimation of milling even if no milling is performed.

[0057] Step 220 may be followed by step 230 of milling each structural element of the array to provide a milled structural element. Each milled structural element has a flat upper surface of a certain width.

[0058] Step 230 may include milling each structural element of the array by scanning a defocused ion beam of a certain width along the longitudinal axis of the structural element. The current intensity of the defocused ion beam decreases with distance from the middle of the defocused ion beam.

[0059] Figure 4 1 is an example of a structural element 111 , a scan line 172 of a scan pattern, and a defocused ion beam current intensity distribution 162 according to one or more embodiments of the present disclosure.

[0060] The scan line 172 is parallel to the longitudinal axis of the structural element 111 (eg, the longitudinal axis 119 ) and is located in the center of the structural element.

[0061] While following these scan lines, the defocused ion beam (having a defocused ion beam current intensity distribution 162 ) “covers” the structural element 111 .

[0062] For each structural element, the total current of the defocused ion beam increases with distance from the vertical sidewalls of the structural element, thereby compensating for the increase in material extraction with decreasing distance from the sidewalls of the structural element.

[0063] Figure 5 is an example of an array 110 of structural elements 111, gaps 115 formed between the structural elements, and a portion of a scan pattern 170 including a scan line 172 and an anti-scan line 171. The scan line 172 is parallel to the longitudinal axis of the structural element 111 and is located in the center of the structural element 111.

[0064] The reverse scan line 171 is located outside the array 110 and is used to "shift" the defocused ion beam between one scan line and another.

[0065] Figure 6 An example of an object 40, such as a semiconductor wafer, is shown, the object 40 comprising an array 110 of structural elements 111 and a focused ion beam module 10 of a milling machine.

[0066] The focused ion beam module 10 includes a beam source 14, a first lens 18, an objective lens 20, and a power supply unit 30. The focused ion beam module 10 is controlled by a controller 34.

[0067] The first lens 18 may be a condenser and / or a scanning lens.

[0068] The objective lens 20 may include a magnetic lens 22 and an electrostatic lens 24. A bias voltage may be applied to the electrostatic lens 24.

[0069] The focused ion beam module 10 may include additional optical elements, other optical elements, or the like.

[0070] The focused ion beam module 10 may generate a defocused ion beam 62 .

[0071] The defocused ion beam 62 may be deflected one or more times.

[0072] Figure 6 An ion beam 62 is shown focused by the objective lens 20 and reaching the upper surface of the object 40 (located in the first plane 51). The focusing plane 52 is below the upper surface of the object 40. The upper surface of the object 40 may be located at the plane of the top surface of the structural element. It should be noted that the focusing plane of the defocused ion beam may be above the upper surface.

[0073] The defocus intensity and other milling parameters may be controlled by the controller 34 .

[0074] The embodiments of the present disclosure may also be implemented in a computer program for running on a computer system, the computer program comprising at least a code portion for executing the steps of the method according to the embodiments of the present disclosure when running on a programmable device (such as a computer system), or enabling the programmable device to perform the functions of the apparatus or system according to the embodiments of the present disclosure. The computer program may cause the storage system to allocate disk drives to disk drive groups.

[0075] A computer program is a series of instructions, such as a special purpose program and / or an operating system. A computer program may include, for example, one or more of the following: a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a server applet, source code, object code, a shared library / dynamically loadable library, and / or other series of instructions designed to be executed on a computer system.

[0076] The computer program may be stored internally on a non-transitory computer readable medium. All or part of the computer program may be provided on a computer readable medium permanently, removably, or remotely coupled to an information processing system. The computer readable medium may include, for example but not limited to, any number of the following: magnetic storage media, including magnetic disks and tape storage media; optical storage media, such as optical disk media and digital video disk storage media; non-transitory memory storage media, including semiconductor-based memory cells, such as flash memory, erasable memory cells, or strong magnetic digital memory; memristors, transient storage media including registers, buffers, or caches, main memory, etc.

[0077] A computer program typically includes an executable (running) program or portion of a program, current program values ​​and state information, and resources used by an operating system to manage the execution of the program. An operating system (OS) is software that manages the sharing of a computer's resources and provides an interface for programmers to access such resources. An operating system processes system data and user input, and responds by allocating and managing tasks and internal system resources as services to users and system programs.

[0078] A computer system may, for example, include at least one processing unit, associated memory, and some input / output (I / O) devices. When executing a computer program, the computer system processes information according to the computer program and generates resultant output information via the I / O devices.

[0079] In the foregoing specification, embodiments of the present disclosure have been described with reference to specific examples of embodiments of the present disclosure. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of embodiments of the present disclosure as set forth in the appended claims.

[0080] Furthermore, the terms "front", "rear", "top", "bottom", "upper", "lower", etc., if any, used in the specification and claims are used for descriptive purposes and are not necessarily used to describe permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the disclosure described herein are, for example, capable of operation in other orientations than shown or described herein.

[0081] Although specific conductivity types or potential polarities have been described in the examples, it should be understood that the conductivity types and potential polarities may be reversed.

[0082] Each signal described herein can be designed as positive or negative logic. In the case of a negative logic signal, the signal is active low, where the logically true state corresponds to a logic level 0. In the case of a positive logic signal, the signal is active high, where the logically true state corresponds to a logic level 1. It should be noted that any signal described herein can be designed as a negative or positive logic signal. Therefore, in alternative embodiments, such signals described as positive logic signals can be implemented as negative logic signals, and such signals described as negative logic signals can be implemented as positive logic signals.

[0083] Additionally, the terms "assert" or "set" and "deassert" (or "de-assert" or "clear") are used herein when referring to causing a signal, status bit, or similar device to assume a logically true or logically false state, respectively. If the logically true state is a logic level 1, the logically false state is a logic level 0. And if the logically true state is a logic level 0, the logically false state is a logic level 1.

[0084] Those skilled in the art will appreciate that the boundaries between logic blocks are merely illustrative, and that other embodiments may merge logic blocks or circuit elements or impose alternative decompositions of functionality on various logic blocks or circuit elements. Therefore, it should be understood that the architectures depicted herein are merely exemplary, and that in fact many other architectures that achieve similar functionality may be implemented.

[0085] Any configuration of components to achieve the same function is effectively "associated" so that the desired function is achieved. Therefore, any two components combined in this article to achieve a specific function can be considered to be "associated" with each other so that the desired function is achieved, regardless of the architecture or intervening elements. Similarly, any two elements so associated can also be considered to be "operably connected" to each other, or "operably coupled" to achieve the desired function.

[0086] In addition, those skilled in the art will appreciate that the boundaries between the operations described above are merely illustrative. Multiple operations may be combined into a single operation, a single operation may be dispersed in additional operations and the operations may be performed at least partially overlapping in time. In addition, other embodiments may include multiple instances of a particular operation, and the order of operations may be changed in various other embodiments.

[0087] For another example, in one embodiment, the examples shown may be implemented as circuits located on a single integrated circuit or in the same device. Alternatively, the examples may be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner.

[0088] As another example, the examples or portions of the examples may be implemented (such as in any suitable type of hardware description language) as software or code that is physical circuitry or a logical representation convertible into physical circuitry.

[0089] In addition, the embodiments of the present disclosure are not limited to physical devices or units implemented in non-programmable hardware, but can also be applied to programmable devices or units that can implement the desired device functions by operating according to appropriate program codes, such as mainframes, minicomputers, servers, workstations, personal computers, notebook computers, personal digital assistants, electronic games, automobiles and other embedded systems, mobile phones and various other wireless devices, which are generally referred to as "computer systems" in this application.

[0090] However, other modifications, changes and substitutions are possible. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

[0091] In the claims, any reference numerals placed between brackets shall not be interpreted as limiting the claims. The term "comprising" does not exclude the presence of other elements or steps other than those listed in the claims. In addition, the term "a" or "an" as used herein is defined as one or more than one. Similarly, the use of introductory words such as "at least one" and "one or more" in the claims shall not be interpreted as implying that any specific claim containing such guided claim elements guided by the indefinite article "a" or "an" shall be limited to an embodiment containing only one such element, even when the same claim includes introductory words "one or more" or "at least one" and indefinite articles such as "a" or "an". The use of definite articles is also the same. Unless otherwise specified, terms such as "first" and "second" are used to distinguish between the elements described by these terms at will. Therefore, these terms are not necessarily intended to indicate the time or other priority of these elements. The pure fact that specific measures are recorded in different claims does not indicate that the combination of these measures cannot be used advantageously.

[0092] Although the specific features of the embodiments of the present disclosure have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those skilled in the art. Therefore, it should be understood that the appended claims are intended to cover all such modifications and changes as falling within the true spirit of the embodiments of the present disclosure.

Claims

1. A method for producing a milled structural element, the method comprising: Milling each structural element in an array of structural elements separated from each other by gaps to provide the milled structural elements; wherein, before the milling, each structural element has a flat upper surface of a certain width, wherein the certain width is nanometer-scale; wherein, after the milling, the upper surface of each structural element is still flat; wherein the milling comprises scanning the defocused ion beam of a certain width along the longitudinal axis of each structural element; and The current intensity of the defocused ion beam decreases with the distance from the middle of the defocused ion beam.

2. The method of claim 1, comprising: Structural element information about the certain width is received, and at least one milling parameter is determined to provide the defocused ion beam of the certain width.

3. The method of claim 1, comprising: receiving or determining a shape of an upper portion of the uniformly milled structural elements of the array; determining a milling plan that compensates for a deviation between the shape of the upper portion of the uniformly milled structural element and having a flat top surface; and During said milling of said each structural element, said milling plan is applied.

4. The method of claim 3, wherein the determining the milling plan comprises determining a milling scan pattern and one or more additional milling parameters, the one or more additional milling parameters comprising at least one of a defocus intensity of the defocused ion beam, or a defocused ion beam current distribution.

5. The method of claim 1, wherein the defocused ion beam has a focal plane above or below the upper surface of the structural elements of the array.

6. A milling machine comprising: Controller; and Focused ion beam module; wherein the focused ion beam module is configured to, under the control of the controller, mill each structural element in an array of structural elements separated from each other by gaps to provide a milled structural element; wherein before milling, each structural element has a flat upper surface of a certain width, wherein the certain width is nanometer-level; wherein after milling, the upper surface of each structural element is still flat; The milling of each structural element by the focused ion beam module comprises: scanning the defocused ion beam of a certain width along the longitudinal axis of each structural element; and The current intensity of the defocused ion beam decreases with the distance from the center of the defocused ion beam.

7. The milling machine according to claim 6, wherein: The controller is configured to receive structural element information about the certain width and determine at least one milling parameter to provide the defocused ion beam of the certain width.

8. The milling machine of claim 6, wherein the controller is configured to: receiving or determining a shape of an upper portion of the uniformly milled structural elements of the array; determining a milling plan that compensates for a deviation between the shape of the upper portion of the uniformly milled structural element and having a flat top surface; and wherein the focused ion beam module is configured to apply the milling scheme during the milling of each structural element.

9. The milling machine of claim 8, wherein the controller is configured to determine the milling plan by determining a milling scan pattern and one or more additional milling parameters, the one or more additional milling parameters comprising at least one of a defocus intensity of the defocused ion beam, or a defocused ion beam current distribution.

10. The milling machine of claim 6, wherein said defocused ion beam has a focal plane below said upper surface of said structural elements of said array.

11. A non-transitory computer readable medium storing instructions for producing a milled structural element, the instructions causing a milling machine to perform a method comprising: Milling each structural element in an array of structural elements separated from each other by gaps to provide the milled structural elements; wherein, before the milling, each structural element has a flat upper surface of a certain width, wherein the certain width is nanometer-scale; wherein, after the milling, the upper surface of each structural element is still flat; wherein the milling comprises scanning the defocused ion beam of a certain width along the longitudinal axis of each structural element; and The current intensity of the defocused ion beam decreases with the distance from the middle of the defocused ion beam.

12. The non-transitory computer readable medium of claim 11, the method further comprising: Structural element information about the certain width is received, and at least one milling parameter is determined to provide the defocused ion beam of the certain width.

13. The non-transitory computer readable medium of claim 11, the method further comprising: receiving or determining a shape of an upper portion of the uniformly milled structural elements of the array; determining a milling plan that compensates for a deviation between the shape of the upper portion of the uniformly milled structural element and having a flat top surface; and During said milling of said each structural element, said milling strategy is applied.

14. The non-transitory computer readable medium of claim 13, wherein the determining the milling scheme comprises determining a milling scan pattern and one or more additional milling parameters, the one or more additional milling parameters comprising at least one of a defocus intensity of the defocused ion beam, or a defocused ion beam current distribution.

15. The non-transitory computer readable medium of claim 11, wherein the defocused ion beam has a focal plane below the upper surface of the structural elements of the array.

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