Tip characterization method using multi-feature characterizer for CD-AFM

Abstract

In atomic force microscopy (AFM) metrology, the tip is a key source of uncertainty. Images taken with an AFM show a change in feature width and shape that depends on tip geometry. This geometric dilation is more pronounced when measuring features with high aspect ratios, and makes it difficult to obtain absolute dimensions. In order to accurately measure nanoscale features using an AFM, the tip dimensions should be known with a high degree of precision. We evaluate a new AFM tip characterizer, and apply it to critical dimension AFM (CD-AFM) tips used for high aspect ratio features. The characterizer is made up of comb-shaped lines and spaces, and includes a series of gratings that could be used as an integrated nanoscale length reference. We also demonstrate a simulation method that could be used to specify what range of tip sizes and shapes the characterizer can measure. Our experiments show that for non re-entrant features, the results obtained with this characterizer are consistent to 1nm with the results obtained by using widely accepted but slower methods that are common practice in CD-AFM metrology. A validation of the integrated length standard using displacement interferometry indicates a uniformity of better than 0.75%, suggesting that the sample could be used as highly accurate and SI traceable lateral scale for the whole evaluation process.

Keywords: Critical dimension atomic force microscope; Tip characterizer; Tip dilation.

Fig. 1

(a) schematic diagram of a tip apex and a surface, and the resulting dilation from the tip. (b) Schematic diagram of the dilation introduced by the tip when measuring a high aspect ratio feature using conical and cylindrical tips.

Fig. 2

Schematic diagram of the CD-AFM operation. The tip vibrates in the Z direction and dithers in the lateral direction. The tip is able to track both the vertical and lateral surfaces by adjusting the servo direction when a change in slope is detected by the sensor. (b) SEM image of a representative 15 nm CD-AFM tip made of high density diamond-like carbon by means of electron beam induced processing.

Fig. 3

Profiles of (a) non-flared and (b) flared tips (c) the apparent profile produced by tip dilation.

Fig. 4

(a) Schematic diagram of the comb tip characterizer, (b) CD-AFM image of the characterizer. Instrument: Insight CD-AFM, scan size: x:4 μm, y:1 μm; z-range: 90 nm. Tip: 50 nm cylindrical CD tip (non-flared); average scan speed: 0.477 Hz (c) TEM image of some of the features.

Fig. 5

Definition of probe width (W₁, W₂) and probe length (L₁, L₂).

Fig. 6

Probe shape characteristic determination using the narrow ridge structure. (a) Profile of the probe feature. (b) Profile of the probe feature showing the portion attributed to the feature width. (c) Profile of the feature after subtracting W_o. (d) Truncated profile of the comb feature showing 90% of the profile. The bottom portion of the profile could contain non tip artifacts. (e) Inverted version of the Fig. 4(d). (f) Probe profile used for analysis.

Fig. 7

(a) Profile of a comb feature with width (W) measurements at specified L locations. (b) Probe asymmetry against x–y plane determined from (a).

Fig. 8

(a) average profile of the CD-AFM image in Fig. 4(b) including the effect of tip dilation. The image was produced by a 50 nm cylindrical CD tip (b) to (e) close-up plots of select features. For each feature, a probe characteristic (shown in Fig. 9) is developed.

Fig. 9

Probe characteristics of all the features shown in Fig. 8(a). The length of the profile depends on the depth of the feature.

Fig. 10

A schematic diagram of a HAR feature, showing where the top, middle, and bottom CDs are calculated, and the sidewall angle.

Fig. 11

(a) Profile of the comb characterizer. (b) Close- up view of the grating portion of the sample. (c) Power spectral density profile of the grating in frequency domain.

Fig. 12

Probe characteristic and reconstructed profiles for non-flared (a–c) and (d and e) flared tips.

Fig. 13

SEM images of the flared tips used in Table 2: (a) Flared CD tip2 (b) corresponding CDR-EBD tip made of diamond-like carbon with a nominal tip width of 40 nm (CDR40-EBD). (c) Flared CD tip3 (d) corresponding CDR-EBD tip with a nominal tip width of 50 nm (CDR50-EBD, right). (a) and (c) were acquired with landing energies of 5 kV, with 43 pA. The images have magnification of 500 k x. (b) and (d) have magnifications of 200 k x.

Fig. 14

(a) Linewidth result for the non-flared tip on (a) etched silicon and (b) resist. (c)Linewidth results for the flared tip on etched silicon.

Fig. 15

(a) TEM micrograph of the comb characterizer. (b) A close-up image of the comb features. (c) Close-up view of one of the features showing the sidewall angle and corner radius. (d) Close-up view of the narrow ridge.

Fig. 16

(a) Simulated profiles for the characterizer, with output for non-flared tips with 18 nm, 32 nm and 46 nm tip widths. (b) Close-up view of line space features. (c) Close-up view of the calibration gratings. (d) Profiles of tip shanks of the simulated tips. The tip width is defined at 10 nm from the tip apex.

Fig. 17

(a) Simulated profiles for the characterizer, with output for flared tips with 10 nm, 15 nm, 18 nm, and 44 nm tip widths. (b) Close-up view of line space features. (c) Close-up of one of the features showing simulated profiles from a 15 nm tip (0 nm VEH) and 15 nm tip (2 nm VEH). (d) Close-up of the simulated profiles from the gratings for 15 nm tip (0 nm VEH) and 15 nm tip (2 nm VEH). (e) flared tip models. Only a portion of the 44 nm tip is shown.