MultiPrep™ Procedure

Thinning of an IC for FIB/TEM G.D. Liechty; C.A. Smith, Allied High Tech Products, Inc. August 2003

Overview Since FIB (focused ion beam) time is valuable, pre-thinning a sample for TEM will dramatically decrease milling time, thus increasing sample throughput and reducing cost of ownership. The MultiPrep™ System accurately thins samples prior to using a FIB system for cross-section SEM or TEM sample preparation. It allows semiautomatic sample thinning with excellent reproducibility, and can simultaneously prepare multiple samples. Samples are typically thinned to a final thickness of 5-20 microns, depending on operator preference. In addition, if one side can be cleaved to within several microns of the area of interest (AOI), only the other side needs to be thinned, improving sample throughput.

In this procedure, an IC approximately 3mm wide is mounted to a fixture and polished within 5 to 10 microns from the AOI. The polished side of the sample is then mounted to a fixture with a Pyrex insert that has been polished parallel with the abrasive plane, and final thinned within 5 to 10 microns from the AOI. The parallel “sliver” can then be milled with the FIB to its final thickness before TEM observation.

It is strongly recommended that the MultiPrep™ System manual be studied to ensure familiarity with the terms used to describe the functions and components in this procedure. Consumable selection, machine settings and techniques used in this procedure were developed using the MultiPrep™ System in Allied’s applications laboratory.

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Equipment Used: 15-2000 MultiPrep™ System 15-1005 Cam-Lock Adapter 15-1010 Cross-Sectioning Paddle 15-1013 TEM Wedge/FIB Thinning Paddle 5-5000 TechCut 4™ Precision Low Speed Saw

Zeiss Axioskop 2 MAT Compound Microscope Zeiss Stemi DV-4 Stereomicroscope Zeiss AxioVision 4 Measurement Software

Note: Part numbers for microscopes and software depend on desired configurations.

Consumables Used: 71-10040 Hot Mounting Wax 71-40045 Loc-Tite 460 72-20000 Glass Cover Slips (optional) 50-30045 15µm Diamond Lapping Film (DLF) 50-30050 9µm DLF 50-30055 6µm DLF 50-30060 3µm DLF 50-30070 0.5µm DLF 50-05518 Rubber Squeegee 90-150-350 Red Final C Polishing Cloth 180-20000 0.05µm Non-Crystallizing Colloidal Silica Suspension 148-10000 Micro Organic Soap 210-30000 Cotton-Tipped Applicators (Swabs) 50-30000 DLF Storage/Blotter Book 200-20000 Aero-Duster Canned Air

Other: Hot Plate w/ Temperature Readout (Dataplate PMC 720) Microscope Slides Tweezers, Sharp End Acetone Isopropyl Alcohol Paper, 8 x 11 White Filter Paper

Note: For some samples, it may be necessary to secure a glass cover slip to the front face (circuit side or film side) to deter delamination from the substrate. This decision will determine which product (wax or glue) is used to secure the sample to the cross-sectioning paddle. If wax is used to secure the cover slip, glue must be used to secure the sample to the paddle. For the purposes of this procedure, no cover slip will be used.

Procedure

1. Calibrate the MultiPrep™ System according to the procedures in the manual. 2. Place the cross-sectioning paddle onto the hot plate and heat to 175°C. 3. Cut or cleave the sample to be polished from a device or wafer. 4. Melt a small amount of wax onto the heated cross-

sectioning paddle near the bottom edge, then remove the paddle from the hot plate.

5. Place the sample onto the paddle with the circuit side up and align the circuit geometry with that of the paddle (see Photo 1). Angle adjustments are made using the micrometers on the MultiPrep™. At least 50% of the sample should be on the paddle and the AOI should extend over the edge. Note: A stereomicroscope with a cross-hair reticle is very helpful when aligning the sample with the paddle.

6. In the microscope, locate the AOI to be cross-sectioned. Photo 1 7. Use AxioVision to measure the distance from the edge of the sample to the AOI. It may be necessary

to use the lowest magnification to fit the edge and AOI into the field of view. A stereomicroscope can also be used in the same capacity if the field of view in the compound microscope is too small.

8. Secure a 15µm DLF to the platen using the rubber squeegee. 9. Attach the cam-lock adapter to the MultiPrep™ and the cross-section paddle to the adapter as shown in

Photo 2.

10. Lower the sample with the spindle riser

(if raised). Then raise the sample using the vertical adjustment knob if necessary so it does not touch the DLF. Zero the digital dial indicator by pressing the yellow button labeled “Zero”.

11. Place a small mirror onto the platen

Photo

1314

1516

Approx. 3.5 mm

#15-1005

under the sample and lower it close to, but not touching, the mirror.

12. Using the “right” side micrometer

#15-1010

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associated with the “roll” adjustment of the sample, rotate it to adjust the sample so the circuit geometry is aligned with its reflection in the mirror. If the sample needs to be rotated clockwise,

2, Sample Position and Platen Direction

rotate the micrometer head clockwise. Conversely, rotate the micrometer counterclockwise if the sample needs to be adjusted counterclockwise. Note: Each vertical mark on the micrometer head thimble represents 0.02°.

. Once the sample and its reflection are aligned, remove the mirror from the platen.

. Lower the sample into the abrasive until the digital dial indicator displays 100 microns more than what needs to be removed. Note: The objective is to stop approximately 200 microns from the AOI after using 15µm DLF. This is measured and verified using AxioVision.

. Raise the sample using the spindle riser.

. Activate platen rotation counterclockwise at 10 RPM.

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17. Activate coolant, gently lower the sample onto the DLF with the spindle riser and zero the dial indicator. 18. Increase platen RPM to 75. When the display indicates that all but approximately 200 microns has been

polished from the AOI, lift the sample from the abrasive with the spindle riser.

Note: The sample holder and sample should be cleaned between steps using water and a cotton-tipped applicator.

19. Clean the DLF with a lint-free wipe and place it into the DLF Storage/Blotter Book. Follow this practice with all DLF.

20. Secure a 6µm DLF to the platen and polish the sample until approximately 100 microns are left to the AOI. Repeat steps 14–19 except use 60 RPM.

21. Inspect the angle of polish to determine if alignment using the micrometer head is necessary. If no angle adjustment is necessary, proceed to step 25.

Photo 3a Photo 3b Note: The sample shown in Photo 3a requires a counterclockwise rotation adjustment. Using AxioVision, the angle is measured so that an exact adjustment can be made. By subtracting the angle (88.87) from 90, the difference is 1.13°. Divide the result by 0.02 (the increment of the micrometer head, in degrees), which equals 56.5. This represents the number of vertical lines (in Photo 4) the micrometer needs to be adjusted. Fifty vertical lines equal one full revolution and 1°.

22. Adjust the micrometer head counterclockwise 56.5 increments.

23. Continue polishing until the sample is polished edge to edge. Note: Observation of the platen and the debris trail width will indicate when this happens. As one edge of the sample is lower, the debris trail will be narrow at first and widen as the sample advances into the abrasive. During this step, a certain amount of material will be removed, determined by the degree of misalignment. If the sample is grossly misaligned, it may require more material removal that may run past the AOI. Therefore, it is important that enough material remains on the sample prior to this adjustment so the AOI is left untouched/intact.

23. Continue to polish and inspect the sample until aligned. 24. Set the sample load to 300 grams (as sample size

varies, the load should be compensated accordingly) Ph

Previous alignment

0.02°

oto 4

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25. Secure a 3µm DLF to the platen and polish the sample until approximately 25 microns of material are left to the AOI. Repeat steps 14-19, except use a platen speed of 40 RPM.

26. Secure a 1µm DLF to the platen and polish all but approximately 10 microns to the AOI. Repeat steps 14-19, except use a platen speed of 20 RPM.

27. Place a microscope slide onto a hot plate set to 175°C. 28. Melt wax onto the slide, place the circuit side of the sample into the wax (Photo 5) and remove it to

cool at room temperature. 29. Secure the slide into the saddle clamp and attach to the arm of the TechCut 4™ (see Photo 6). Rotate

the slide in the fixture so the circuit geometry is aligned with the edge of the blade. 30. Position the sample by adjusting the micrometer so the blade is approximately 1,000 microns from the

polished edge. 31. Section through the sample but not through the microscope slide (see Photo 7).

Photo 6

Photo 5 Photo 7

32. Place the slide onto the hot plate. Once the wax is melted, remove the smaller sample and place it in a

beaker of Acetone. 33. Swirl the sample thoroughly to remove the traces of wax. 34. Remove the sample from the Acetone and place it onto filter paper to absorb the Acetone. 35. Place the sample in a beaker of IPA (isopropyl alcohol) and swirl it for 20 to 30 seconds to remove the

residual Acetone. 36. Remove the sample and place it onto filter paper to absorb the excess IPA. 37. With a cotton swab, clean the sample using a solution of Micro Organic Soap mixed with water (1:10). 38. Rinse the sample with water and dry it using a clean air source such as an Aero-Duster. 39. Secure a 9µm DLF to the platen. 40. Using the cam-lock adapter, attach the TEM/Pyrex paddle onto the MultiPrep™ configured as shown in

Photo 14. 41. Lower the spindle with the spindle riser (if raised) and raise the arm using the vertical adjustment knob

so the Pyrex does not touch the DLF. Zero the digital dial indicator by pressing the yellow button labeled “Zero”.

42. Activate platen rotation counterclockwise at 100 RPM. 43. Activate coolant and lower the paddle (using the vertical adjustment knob) until the Pyrex makes

contact with the DLF and the digital dial indicator displays at least 75 microns. Be sure to polish enough of the Pyrex so the entire surface is parallel with the platen.

44. Repeat steps 41-45 using 3µm DLF, and polish the Pyrex for at least 3 minutes at 80 RPM.

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45. Raise the spindle with the spindle riser. 46. Remove the paddle and clean the polished surface of the

Pyrex with IPA; dry using clean air. 47. Mount the sample to the Pyrex using either wax or glue.

There are advantages with each method and only with trial and error will it be possible to determine the better of the two.

#15-1013 TEM Paddle

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Glue method: Loc-Tite 460 is recommended for mounting the sample using this method. It is a very reliable and consistent product, more so than other similar products referred to as “Super Glue” or “nail” glues like “Sally Hansen’s.” It is soluble in Acetone and requires no heat to cure.

A. Position the TEM paddle under the stereomicroscope as shown in Photo 8.

B. Place the polished edge of the sample onto the polished Pyrex just behind the front edge until ready to glue it into position.

C. Using the end of a toothpick or cotton-tipped applicator (wood end), apply a small amount of Loc-Tite 460 across the front, polished edge of the Pyrex.

D. With tweezers, lift, place and position the sample as shown in Photo 9.

E. Using firm but gentle pressure, press the sample against the Pyrex and squeeze the glue to produce a thin glue line. The sample should be mounted as close to and parallel with the front edge of the Pyrex as possible.

F. Allow enough time for the glue to harden. Photo 8 Note: Using too much glue will prolong the cure time. Observe the glossy nature of the glue surface to determine if it is sufficiently hardened to continue. This may require remounting the sample a few times until the technique and amount of glue used is optimized.

G. Once the glue is cured, measure the sample using AxioVision (Photos 10 and 13). Be sure to measure the thickest side of the sample if it is cut unevenly.

Proceed to step 48 at this time.

Photo 9 Photo 10

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Wax method: This method requires the use of a hot plate. A stereomicroscope is used for viewing the sample when mounting it to the Pyrex. Note: Position the stereomicroscope close to the hot plate to shorten the distance when transferring the heated fixture. A heating block (#15-1017) is used to prolong the heat during the transfer to the Pyrex so the wax will remain liquid during mounting (see Photo 11).

A. Place the TEM paddle and the heating block onto the hot plate. B. Place a microscope slide onto the hot plate at 175° C and melt a small amount of wax (a bead) on the

slide (see Photo 12). C. Using tweezers, lift the cleaned sample and place the polished side of the sample into the wax. D. Lift the sample from the bead and place it next to the bead. Allow the wax to flow from the sides of

the sample for about 10 seconds. E. Repeat about 3 times to new locations on the slide to remove the excess wax. F. Place the paddle onto the heating block and transfer the block to the stereomicroscope. G. With tweezers, lift and position the sample as shown in Photo 9 and press it into the Pyrex to compress

the wax from between the sample and the Pyrex. The sample should be mounted as close to and parallel with the front edge of the Pyrex as possible.

H. Remove the paddle from the heating block to cool for approximately 5 minutes. Once the wax has hardened, measure the sample using AxioVision. Be sure to measure the thickest side of the sample if the sample is cut unevenly.

Proceed to step 48 at this time.

Photo 12

Photo 11

Wax Bead

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48. Secure a 15µm DLF to the platen. 49. Raise the arm more than the thickness of

the sample using the vertical adjustment knob so that when the spindle is lowered with the spindle riser the sample does not touch the DLF.

50. Attach the TEM paddle onto the cam-lock adapter (see Photo 14).

51. Position the sample between the edge and the center of the platen (see Photo 14).

52. Lower the spindle riser and zero the dial indicator.

53. Using the vertical adjustment knob, lower the sample into the abrasive until the digital dial indicator displays 100 microns more than what needs to be removed. For example, if 800 microns are to be removed, the dial should display “900” Photo 13 microns. Note: The objective is to stop approximately 200 microns from the AOI using 15µm DLF.

54. Set load to 300 grams. 55. Raise the sample using the spindle riser. 56. Activate platen rotation at 10 RPM counterclockwise. 57. Activate coolant and gently lower the sample into the DLF using the spindle riser. Once contact is made

between the sample and the abrasive, zero the dial indicator. Increase platen speed to 75 RPM, polish all but approximately 200 microns from the sample and raise the sample with the spindle riser.

58. Clean and measure the sample. Note: It may be necessary to increase the magnification to get a more accurate measurement. The polished edge will now be parallel with the Pyrex.

59. Secure a 6µm DLF to the platen. 60. Polish the sample until approximately 100 microns

thick. Follow steps 56 and 57 using 60 RPM. 61. Stop the platen and remove the sample for

measurement and inspection. 62. Secure a 3µm DLF to the platen. 63. Polish the sample until it is between 10-20

microns thick. Follow steps 56 and 57 using 60 RPM.

The sample may now be secured to a mounting stub or foil for the FIB. Photo 14

Approximately 200 microns

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Equipment Photo Page

Stemi DV-4

TechCut 4

MultiPrep System™

Axioskop 2 MAT

AxioVision 4™ Imaging/Capture Software