# Patent

## Title
Test structures for measuring near field transducer disc length

## Abstract

Systems and methods for using NFT disc test structures for controlling NFT disc length during manufacture of an HAMR writer are disclosed. An NFT is manufactured concurrently with one or more pairs of pin-disc and disc-less test structures. The NFT disc and pin dimensions may be substantially similar to the pin and disc dimensions of the pin-disc test structure. The disc length of the pin-disc test structure is measured as a function of the difference in resistance between the two test structures and other parameters. Capturing the disc length variation subsequently enables adjustment of the NFT electronic lapping guide stripe height to reduce length variation in the NFT pin.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Application Ser. No. 61/888,467, filed Oct. 8, 2013, which is incorporated herein by reference in its entirety.


BACKGROUND
Heat-assisted magnetic recording (HAMR) writers have been developed to meet the growing demand for improved magnetic disk drive data capacity. HAMR writers heat high-stability magnetic compounds to apply changes in magnetic orientation. These materials can store bits in a much smaller areas without being limited by the superparamagnetic effect. The manufacture of HAMR writers may be improved by carefully controlling the lapped pin length, the distance from the air-bearing surface (ABS) to the intersection of the Near-Field Transducer (NFT) pin and NFT disc (or disk).



BRIEF DESCRIPTION OF THE DRAWINGS
The present application is illustrated by way of example, and not limitation, in the figures of the accompanying drawings in which:
FIGS. 1A-1B illustrate an exemplary HAMR writer that may be manufactured in accordance with embodiments of the present disclosure.
FIG. 2 is an operational flow diagram illustrating an exemplary process for measuring the disc length of an NFT during manufacture of an HAMR writer.
FIG. 3 illustrates an example pair of resistor test structures that may be used to measure the disc length of an NFT.
FIG. 4 illustrates an example pair of linear series resistor test structures that may be used to measure the disc length of an NFT.
FIGS. 5A-5B illustrate an example pair of serpentine series resistor test structures that may be used to measure the disc length of an NFT.
FIG. 6 illustrates an example pair of near four-point test structures that may be used to measure the disc length of an NFT.
FIG. 7 illustrates an example pair of true four-point test structures that may be used to measure the disc length of an NFT.



DETAILED DESCRIPTION
In the following description, numerous specific details are set forth to provide a thorough understanding of various embodiment of the present disclosure. It will be apparent to one skilled in the art, however, that these specific details need not be employed to practice various embodiments of the present disclosure. In other instances, well known components or methods have not been described in detail to avoid unnecessarily obscuring various embodiments of the present disclosure.
In accordance with the present disclosure, systems and methods for using NFT disc test structures for controlling NFT disc length during manufacture of an HAMR writer are disclosed.
FIGS. 1A-1B illustrate an exemplary HAMR writer 100 that may be manufactured in accordance with embodiments of the present disclosure. FIG. 1A is a cross-sectional view of HAMR writer 100. HAMR writer 100 may comprise a waveguide 112, a pole 114, a near-field transducer (NFT) 116, a grating 120, and a light (e.g. laser) spot 122 on the grating 120. FIG. 1B is a top view of NFT 116. NFT 116 includes a disc portion 116B and a pin portion 116A. Disc portion 116B may be shaped as a circle, a square, or another shape. The light or light energy from light spot 122 on grating 120 is coupled to waveguide 112, which guides the light energy to NFT 116 near air-bearing surface (ABS) 115. Disc portion 116B of NFT 116 collects light energy from waveguide 112 and radiates it through pin 116A to media 130 on spot 132 to elevate the temperature of media 130 and reduce coercivity and change the magnetization of the media. HAMR 100 may then write data to the heated region of recording media 130 by energizing pole 114.
During manufacture of HAMR 100, it is desirable to control dimensions of NFT 116, particularly the length of pin portion 116A. The length of pin portion 116A may be controlled by controlling the length 117 of disc portion 116B during manufacture of HAMR 100. Capturing the disc length variation can enable adjustment of NFT 116 electronic lapping guide (ELG) stripe height to reduce the length variation of pin portion 116A. As further described below, NFT disc test structures manufactured on the same wafer as NFT 116 may be used for controlling NFT 116 disc length. In the test structures, disc length variation may be captured by measuring disc length from the difference as a function of the resistance of two or more test structures.
FIG. 2 is an operational flow diagram illustrating an exemplary process 200 for measuring the disc length of an NFT 116 during manufacture of an HAMR writer 100. For simplicity, some process operations may be omitted. At operation 210, manufacture of an HAMR 100 begins on a wafer. During a subsequent process operation 220, the HAMR NFT 116 is formed. As illustrated in exemplary process, 200, a first test structure comprising a disc and pin and a second disc-less test structure comprising a pin may be concurrently manufactured with NFT 116 on the same wafer at process operations 230 and 240. In this embodiment, the first and test structure are manufactured on the same wafer as NFT 116, for example, isolated on a separate bar at the top of the flash field of the wafer. In other embodiments, the first and second test structure may be manufactured on other parts of the wafer. In one embodiment, the first and second test structure are formed at the same time that the disc and pin of NFT 116 are defined, for example, using the same mask or masks. In some embodiments a plurality of first and second test structure pairs with different dimensions are manufactured concurrently with the manufacture of NFT 116.
Concurrent manufacture of the first and second test structures on the same wafer as NFT 116 provides the benefit of insuring that process variations in the manufacture of HAMR writer 100 and NFT 116 do not influence any data (further described below) obtained from the test structures. For example, concurrent manufacture on the same wafer helps insure that the first test structure disc length is approximately the same as NFT 116 disc length. In this embodiment, a multiple pattern masking structure with a single image of the disc and pin may be used during manufacture of the first and second test structures. In alternative embodiments, the first and second test structures are not manufactured concurrently with NFT 116.
FIG. 3 illustrates an example pair of resistor test structures 310 and 320 that may be manufactured concurrently with NFT 116 at operations 220-250 and used to measure the disc length of NFT 116. Test structure 310 is a disc-less structure comprising a pin 311 and test structure 320 comprises a pin 321 and disc 322. As illustrated in FIG. 3, disc 322 has a different arrangement than on NFT 116, i.e. it is recessed above the ABS. In one embodiment test structure 310 and test structure 320 are defined on the same film (e.g. a gold film) as the NFT 116 disc and pin. In this embodiment, disc 322 and pin 321 (or pin 311) may have approximately the same dimensions as the disc and pin of NFT 116. As illustrated in FIG. 3, test structure 310 and test structure 320 may be defined with approximately the same dimensions (e.g. shape, pin length, etc.). In alternative embodiments, test structure 310 and test structure 320 are defined with different dimensions. Disc 322 and pin 321 of test structure 320 may be defined on different films (e.g. a disc film and a pin film) or the same film (e.g. a pin film).
At operation 250, the resistances of each of the two disc test structures (e.g. test structures 310 and 320) are measured. The sheet resistance of the disc film and pin film are also measured at operation 250 by building a neighboring test structure such as, for example, a van der Pauw test pattern. In embodiments where the disc and pin are on the same film, one sheet resistance is measured. The measured resistances may be used with other parameters to calculate or measure the disc length of the disc-pin test structure. Because the disc-pin structure preferably has approximately the same disc and pin dimensions as NFT 116, the disc length of NFT 116 is measured or estimated in this way. As illustrated in FIG. 3, resistance Ro may be measured for test structure 310 and resistance Rd may be measure for test structure 320 by placing a pair of leads on a pair of contacts 313 and 323, respectively, at the top and bottom of the structures. In some embodiments, the use of contacts 313 and 323 and leads introduces a leads and contact resistance term into the measurements. The precise placement of contacts 313 and 323 may vary in different embodiments of the disclosure.
With the measured resistances of the first and second test structure, at operation 260 the effective disc length of the disc-pin test structure may be determined based on the measured resistances and other parameters. The measured resistance of a test structure that includes one or more pins, one or more discs, and one or more lead structures may be defined as a function that comprises one or more of the following parameters: pin length (Lp), pin width (W), disc diameter (D), disc length (Ld), sheet resistance of pin film (Rs), sheet resistance of disc film, resistance of the disc structure, and total leads resistance. Using this relation, the measured resistances, and other considerations, discussed below, the disc length (Ld) may be defined as a function of known or measured parameters.
In embodiments where the disc-less test structure and pin-disc test structure are approximately identical (e.g. having approximately the same pin length and pin width) and approximately adjacent, the leads resistance term may be eliminated from consideration when defining Ld as a function of the difference in resistance between two adjacent test structures. This comparison carries the benefit of eliminating effects of variation in the pin width in the regions nearest each end of the pin.
In the embodiment where the disc and pin of the pin-disc test structure are formed in a common film, the disc length (Ld) may be described as a function of pin width (W) and disc diameter (D) described by Equation (1):

Ld=(D2−W2)0.5  (1)

In another embodiment, the pin may be defined in a separate resistive film from the disc. In particular implementations of this embodiment where the sheet resistance of the pin film is substantially greater than the sheet resistance of the disc film, the test structure may have a greater sensitivity to variation in D and the estimated Ld may be greater than in the embodiment where the pin and disc are defined on the same film.

In the embodiment where the disc and pin are formed in a common film, the number of unknowns is reduced by one (i.e. only need to measure the single film sheet resistance). The pin length (Lp) of a test structure may be estimated based on the high aspect ratio (e.g. 20:1 or greater) of current masks used to define the pin. The pin width (W) may then be estimated for a structure from a 4-point measurement of the resistance of the pin structure with no disc (Ro), the estimated pin length (Lp) and the measured sheet resistance of the pin film (Rs).
Using the above described considerations the disc length (Ld) may be described as a function of measured or estimated unknowns. An exemplary implementation of this function is illustrated by Equation (2):

Ld=((Ro−Rd)−kRs)/(Rs/W)  (2)

Where:
Ro=measured resistance of the disc-less pin structure
Rd=measured resistance of the pin-disc structure
k=number of squares in nominal disc
W=pin width
Rs=pin film sheet resistance
The parameter k may be determined using finite element modeling (FEM) techniques.
Table 1, below, illustrates finite element modeling (FEM) estimates for particular implementations of both the disc over pin embodiment and the integrated disc and pin embodiment for the resistor test structures 310 and 320 of FIG. 3. In both embodiments, the resistance contribution of the disc is ignored. In the disc over pin embodiment, the pin film comprises Chromium (Cr) and the disc film comprises Gold (Au). In the integrated disc-pin embodiment, the single film comprises Gold (Au). FEM estimates are compared against the calculated disc length from Equation (2).















TABLE 1





Lead Material
Cr
Cr
Cr
Cr
Au
Au
Au
Au























Lead Thickness (nm)
20
20
20
20
200
200
200
200


Pin Material

Au
Au
Au

Au
Au
Au


Pin Type

Overlap
Overlap
Overlap

Integrated
Integrated
Integrated


Disc Diameter/Length (nm)

180
200
220

180
200
220


Pin Width
0.02



0.02





Pin Thickness (nm)

200
200
200

200
200
200


Resistance (Ohms)
716.181
599.146
584.328
569.085
14.319
12.328
12.048
11.765


Pin Rs
15



0.3





Delta Squares

7.80233
8.79018
9.80640

6.63414
7.56860
8.51162


Estimated Ld (from Delta Squares)

0.15605
0.17580
0.19613

0.13268
0.15137
0.17023


Calculated Ld (from modeled disc

0.17889
0.19900
0.21909

0.17889
0.19900
0.21909


dia. & pin width)





Notes:


Bias was applied on a 20 nm width strip each side






In an alternate embodiment, the resistance contribution of the disc is included. In this embodiment, the disc resistance may be estimated from the adjacent film sheet resistance measurement. Table 2, below, illustrates FEM estimates for particular implementations of the integrated disc and pin embodiment for the resistor test structures 310 and 320 when the estimated disc resistance is factored into the calculation of the change in pin resistance/length. In the illustrated embodiment, the disc “squares” constant may be derived using FEM. FEM estimates are compared against the calculated disc length from Equation (2).











TABLE 2





Lead Material
Au
Au
Au
Au







Lead Thickness (nm)
200
200
200
200


Pin Material

Au
Au
Au


Pin Type

integrated
integrated
integrated


Disc Diameter/

180
200
220


Length (nm)






Pin Width
 0.02





Pin Thickness (nm)

200
200
200


Resistance (Ohms) 
 14.319
 12.328
 12.048
 11.765


Disc “squares” @

 2.347
 2.347
 2.347


200 nm






Adjusted Resistance

 11.624
 11.344
 11.061


Pin Rs
 0.3





Delta Squares

 8.98084
 9.91530
 10.85832


Estimated Ld (from 

 0.17962
 0.19831
 0.21717


Delta Squares)






Calculated Ld (from 

 0.17889
 0.19900
 0.21909


modeled disc dia. &






pin width)





Notes:


Bias was applied on a 20 nm width strip each side






FIG. 4 illustrates an example pair of linear series resistor test structures 410 and 420 that may be used to measure the disc length of an NFT in an example embodiment of the disclosure. In this embodiment, disc-pin test structure 421 comprises a plurality of discs provided over an elongated pin structure 422. This configuration provides the benefit of increasing the resistance offset with respect to noise in the leads measurement. In one particular implementation of this embodiment, the plurality of discs in the structure are separated by four times or greater the mask disc design diameter. In one mathematical implementation of this embodiment, the disc length may be estimated by Equation (3):

Ld=((Ro−Rd)−knRs)/(nRs/W)  (3)

Where:

Ld=disc length

Ro=resistance of the disc-less pin structure

Rd=resistance of the pin-disc structure

n is the number of disc elements in the series resistor

Rs=sheet resistance of the NFT film

W=pin width

k=# squares in nominal disc

This embodiment may provide the benefit of a larger resistance difference between the two structures, smaller percentage error in the estimate of pin length, and smaller error in the estimate of pin width.

FIGS. 5A-5B illustrate an example pair of serpentine series resistor test structures 510 and 520 that may be used to measure the disc length of an NFT in an example embodiment of the disclosure. Pin-disc test structure 520 comprises a plurality of discs 521. In this embodiment, the interdependence of pin length and disc separation are avoided by arraying the pins and discs to be connected in a serpentine pattern. In particular embodiments of the serpentine series resistor test structures, the pin length may be process limited.
FIG. 6 illustrates an example pair of near four-point test structures 610 and 620 that may be used to measure the disc length of an NFT in an example embodiment of the disclosure. Pin-disc test structure 620 comprises disc 621. In this embodiment, the four-point resistor test structure configuration eliminates some or all of the contact resistance and nearly all the leads resistance from the measurements. When a difference in length between two near four-point test structures 610 and 620 is well known, two four-point structures of different length may be used so that the residual leads resistance term may be eliminated in calculating the difference in resistance between the two test structures. In particular embodiments of the near four-point test structures, the pin length may be process limited.
FIG. 7 illustrates an example pair of true-four point test structures 710 and 720 that may be used to measure the disc length of an NFT. Pin-disc structure 720 comprises disc 721. The use of true four-point differential test structures 710 and 720 provides the benefit of eliminating contact resistance and lead resistance from the measurements. In particular embodiments of the true four-point test structures, the pin length may be process limited.
In the foregoing specification, embodiments of the disclosure 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 disclosure. The specification and figures are, accordingly, to be regarded in an illustrative rather than a restrictive sense.

## Claims

1. A system for controlling the disc length of a Near Field Transducer (NFT), comprising:
a first test structure comprising a disc and a pin; and
a second test structure comprising a pin;
wherein the first test structure disc length is determined based on a measured resistance of the first test structure and a measured resistance of the second test structure; and
wherein the first test structure disc length is approximately the same as the NFT disc length.

2. The system of claim 1, wherein the first test structure, the second test structure, and the NFT are formed on a same wafer.

3. The system of claim 1, wherein the second test structure is approximately the same length and shape as the first test structure.

4. The system of claim 3, wherein the first test structure and the second test structure each comprise two contacts and two leads.

5. The system of claim 4, wherein the resistance of the first test structure depends on the pin length, the pin width, disc diameter, disc length, and total leads resistance.

6. The system of claim 5, wherein the first test structure disc and pin are defined on the same disc film.

7. The system of claim 5, wherein the first test structure disc is defined on a disc film, and wherein the first test structure pin is defined on a pin film.

8. The system of claim 5, wherein the resistance of the first test structure further depends on the sheet resistance of the disc film and the sheet resistance of the pin film.

9. The system of claim 5, wherein the first test structure and the second test structure are single resistor test structures.

10. The system of claim 5, wherein the first test structure and the second test structure are linear series resistor test structures.

11. The system of claim 5, wherein the first test structure and the second test structure are serpentine series resistor test structures.

12. The system of claim 5, wherein the first test structure and the second test structure are near four-point test structures.

13. The system of claim 5, wherein the first test structure and the second test structure are true four-point test structures.