Automated active feedback slice and view milling of magnetic head cross-sections

A dual/beam FIB/SEM system and method for operating such a system are provided. A micrograph of a throat height view of a magnetic writer is obtained through iterative milling and repeated evaluation of the leading bevel angle or pole length. In some cases, the milling depth for a next iteration may be modified based on evaluation of the leading bevel angles of the current iteration.

TECHNICAL FIELD

This invention relates to the field of metrology and more specifically, to subsurface metrology of magnetic write heads.

BACKGROUND

The performance of many devices fabricated using semiconductor methods is critically dependent upon the three-dimensional (3D) structure thereof. For example, the performance of a perpendicular magnetic recording (PMR) write pole is highly dependent upon the sub-surface shape of the write pole under the air bearing surface (ABS). To obtain information about the efficacy of manufacturing methods of these and other devices, it is desirable to perform metrology on micrographs of cross sections of the write pole at various orientations. One such desired cross section micrograph is perpendicular to the leading edge near the center of the pole. This cross section allows metrology on various write pole characteristics, such as the leading edge bevel and throat height.

Typically, cross sections are obtained by milling a device near a desired cut location, and then obtaining a micrograph of the milled surface. For example, dual beam focused ion beam, scanning electron microscope (FIB/SEM) systems are often used for cross sectional metrology. Such systems can perform milling operations, generate micrographs, and deliver cross-sectional metrology information. However, proper cut placement is necessary to obtain suitable metrology information.

The conventional approach to cross-section devices in magnetic recording heads for sub-surface metrology measurements on the devices involves the following steps: First, a low/medium magnification image of the feature of interest—for example, the ABS—for positioning and alignment is obtained. Second, fiducial markers are processed for position referencing. Third, the device is ion beam milled in proximity to the fiducial markers using fixed, pre-defined milling parameters. Finally, imaging and metrology measurements are performed on the final cut face surface.

Often, especially during research and development stages, this inflexible approach provides insufficient efficiency and accuracy. For example, there may be multiple designs per wafer, per section, or per rowbar; devices may have different geometries within a wafer, a section, or a rowbar; and immature process may have intrinsic process variations. These variations often result in widely variable geometries with very tight dimensional windows for the final cut face surface placement which allow accurate metrology in the final image.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth, such as examples of specific layer compositions and properties, to provide a thorough understanding of various embodiments of the present invention. It will be apparent however, to one skilled in the art that these specific details need not be employed to practice various embodiments of the present invention. In other instances, well known components or methods have not been described in detail to avoid unnecessarily obscuring various embodiments of the present invention.

FIG. 1Aillustrates a view of a PMR writer pole104at the ABS. This view is indicative of what might be seen when viewing a SEM of the writer pole taken at the ABS. The writer pole comprises the magnetic writer pole region102. The pole102is surrounded by a non-magnetic leading bevel107and a trailing edge100. The pole is surrounded by a magnetic shielding material105. The pole104has a pole width at the trailing edge101, representing the width of the magnetic writer pole102at the trailing edge of the writer104. In some cases, the pole104has a pole width at the leading edge106of the magnetic pole material102. The pole104also has a thickness or length103that is the distance between the leading edge and trailing edge of the writer102.

FIG. 1Billustrates a throat height view of the PMR writer pole104. The leading leading bevel angle109is the angle of the leading edge of the leading bevel107. In the illustrated embodiment, the angle109is defined with respect to the line111perpendicular to the throat height. However, the angle109may be defined with respect to other axes and in other manners. The trailing leading bevel angle110is the angle of the trailing edge of the leading bevel107.

FIG. 2Aillustrates an ABS view of a writer203that is unsuitable for a conventional blind milling process whileFIG. 2Billustrates an ABS view of a writer204that is suitable for a conventional blind milling process. The width of the write pole201defines a region205in which a throat height cross section is desired for metrology. If the write pole has a trapezoidal shape, as with write pole205, the cross sectional region206may be defined as the region within the width of leading edge. If the write pole has a triangular shape, as with write pole201, the desired region205may be within some threshold of the leading apex. For example, the desired region20may be between the ⅓, ⅔, or some other length of the two leading edges.

FIG. 3illustrates a method for obtaining a throat height view micrograph suitable for metrology on a magnetic writer. In step301, a micrograph, such as a SEM, of the writer at the ABS is obtained. For example, step301may comprise using a dual-beam FIB/SEM system to obtain an ABS view of a write pole. The micrograph obtained in step301has sufficient resolution to measure the width of the pole at the ABS and to measure the length of the pole at the ABS.

In step303, some initial metrology is performed on the micrograph obtained in step301. In the illustrated method, measurements of the width of the pole at the ABS and measurements of the length of the pole at the ABS are performed. For example, the width of the pole at the ABS may be the width of the pole at the trailing edge of the pole. In other cases, the width of the pole at the ABS may be the leading edge width, or a width at a predetermined position along the length of the pole. Based on the results of step303, the method proceeds to an iterative milling and imaging process304or a conventional blind milling process306. For example, the method may proceed to the conventional blind milling process306if the width is greater than 100 nm. In this example, the method may proceed to the iterative process304if the width is less than 100 nm. In some cases, a minimum width is required for any metrology on the throat height. In these cases, the method may end without any cross sectioning307if a minimum width is not obtained. For example, if the width is less than 55 nm, the method may end307.

Additionally, metrology and decisions304may be performed based on writer height. For example, the height is greater than a threshold, the process may proceed to the conventional blind milling process306. If the height is less than threshold (and possibly greater than a minimum height threshold) the method proceeds to the iterative process304. In some cases, both the height and width are measured and must be within threshold ranges for the method to proceed to the conventional blind milling process306or the iterative process304.

In the iterative process304, a first cross section is made of the writer. The location of the initial milling region is proximal to an outer edge of the pole. The cross section is then imaged to form a micrograph of the milled surface. The method then proceeds to step305to perform metrology on the micrograph.

During step305, the length of the pole is obtained from the micrograph obtained in step304. The length of the pole measured in step304is then compared to the length of the pole obtained in step303. The measurement from step304being approximately equal to the length obtained in step305is an indication that the micrograph obtained in step304is suitable for further metrology.

Additionally, during step305, the leading leading bevel angle and the trailing leading bevel angle are measured on the micrograph obtained in step304. Convergence of the leading leading bevel angle and the trailing leading bevel angle is a second indication that the micrograph obtained in step304is suitable for further metrology.

In the illustrated method, if the difference between leading bevel angles are below a bevel angle threshold and if the difference between the pole length obtained at the ABS in step303and the pole length at the throat height view obtained in step304are below a pole length threshold then the method ends307and provides the image obtained in step304as the final image for subsequent metrology. If either the difference between bevel angles is greater than the bevel angle threshold or the difference between pole lengths is greater than the pole length threshold, then the method reiterates from step304. In a specific example, the bevel angle threshold is 4° and the pole length threshold is approximately 0 (i.e., the pole lengths must be approximately equal to proceed to step307). In other examples, the bevel angle threshold or pole length threshold may be other values. For example, the pole length threshold could be expressed as some percentage, such as 10%, of the nominal initial dimension measured in the ABS micrograph.

In the illustrated method, the mill rate (i.e., the depth of the next milling iteration) is adjusted302based on either the current iteration's difference between the leading bevel angles or difference between the pole lengths. For example, the milling rate may be based only on the difference between the leading leading bevel angle and the trailing leading bevel angle. In a particular example, the milling rate is: (a) 3 nm if the difference is greater than 15°; (b) 1 nm if the difference is between 8° and 15°; or (c) 0.5 nm if the difference is between 4° and 8°.

Subsequent iterations proceed as described above with respect to the first iteration. Step304is repeated for each iteration, such that the sample is milled (using the depth obtained from step302) and the milled surface is imaged. The metrology step305is repeated and, based on the results, the method ends307or another iteration 302, 304 is performed.

In other methods, the method may end307if only one of the conditions described with respect to step305is met. In further methods, different iterations may evaluate different conditions. For example, in a first iteration, only the pole lengths are evaluated in step305, and in further iterations only the bevel angles are evaluated. Such a method may be able to better accommodate the triangular writer poles, where convergence of the pole lengths is unlikely.

FIGS. 4-6illustrate possible milling locations and iterative micrographs as obtained during a series of iterations of a method performed in accordance withFIG. 3.

FIGS. 4A and 4Billustrate an initial milling location and a potential micrograph obtained at the first iteration.FIG. 4Aillustrates the milling location404on an ABS view401of the pole. The first milling location404is proximal to the outer edge402of the pole. Here, the milling location404intersects the outermost corner405of the pole. However, in other cases, the initial milling location404may be at any location proximal to edge402. For clarity, the portion of the pole to the right of the milling location404is still illustrated, however, one of ordinary skill will understand that this region is removed during milling.

FIG. 4Billustrates a micrograph that might be obtained after milling at location404. As illustrated, the pole does not impinge the ABS408at location404, so there is no pole length to measure. Additionally, the leading leading bevel angle406and trailing leading bevel angle407are substantially different from each other. Both of these factors indicate that this micrograph would be unsuitable for subsequent metrology.

FIG. 5Aillustrates a second milling location501at the ABS view401. As illustrated, the milling location501is closer to the center of the pole than the location402.FIG. 5Billustrates a possible micrograph that might be obtained at this location. As illustrated, the pole impinges the ABS502at this location501, resulting in a measurable pole length505. Additionally, the angles503and504are closer to each other. However, in this micrograph, the length505has not converged to the length measurable at the ABS401and the angles503and504are not within the angle threshold. Accordingly, the micrograph in5B would still not be suitable for subsequent metrology.

FIG. 6Aillustrates another iterative milling location601. For example, in the method ofFIG. 3, location601may be milled after milling at location501. In this step, the milling location601has reached the leading edge602of the pole. A potential resulting micrograph is illustrated inFIG. 6B. In this iteration, the length604has converged to the length measurable from the ABS view401and the angles605and603have converged to within the angle threshold. Accordingly, the micrograph ofFIG. 6Bwould be suitable for subsequent writer metrology.

FIG. 7illustrates an example dual-beam FIB/SEM system in which the methods described herein may be implemented. The system700includes a controller701, a workstation702, a FIB703and a SEM704. The controller701operates the FIB703to mill the sample705at selected locations. The controller701further operates the SEM704to image the sample705. For example, the system700may be able to obtain images of the sample705at the ABS and at the milling planes created by the FIB703. The controller701is further coupled to a workstation702that allows a system operator to input control programs and to manually control the operation of the system700. For example, the controller701can comprise non-transitory computer readable medium that can be provided with program code for executing the methods described herein.

In the foregoing specification, embodiments of the invention have been described with reference to specific exemplary features thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and figures are, accordingly, to be regarded in an illustrative rather than a restrictive sense.