**3. Results and discussion**

where the electron and ion column are placed vertically (at 0°) and at an angle 52°, respectively. Alternatively, the same viewing angle of another gun can be obtained

Our Fabrication of MWCNT based AFM probe comprises of four steps. Firstly, using electron beam induced deposition (EBID) hydrocarbon, CNT is welded on the etched part of tungsten tip (**Figure 3(b)**). The carbon atoms are deposited at the interface of tungsten which makes the CNT to attach firmly with tungsten. Secondly, CNT is aligned in the FIB system through the ion beam bending (IBB) [27] (**Figure 3(c)**). FIB is irradiated from the top. In the complete process of CNT alignment, acceleration voltage was set to 30 kV, resolution was 512 pixels 442 pixels and FIB image magnification was 5000. Beam current as well as acceleration voltages can be increased to reduce the alignment time, however, should be optimized to avoid sample damage. We have used this method on 110–170, 50–80 and 20 nm CNT and it has been found that exposure time (for alignment process) to MWCNT is directly proportion to the diameter of MWCNT. For example, MWCNT with the diameter 20 nm takes less exposure time as compared to 50–80 or 110– 170 nm MWCNT. The variables that affect the alignment process includes FIB magnification, exposure time, beam current and accelerating voltage. To produce good welding, many combinations can be made that produce good results. On the

*Fabrication of MWCNT AFM probe. (a) As attached using DEP, some impurities are also attached with MWCNT which can be removed using lower current ion beam. (b) After hydrocarbon deposition (welding) on the interface of MWCNT and Tungsten probe. (c) Alignment of MWCNT using IBB process (d) using gallium ion bombardment extra length of CNT is cut off. To give some offset from the sidewall (in case of AAO sample or samples having vertical side walls), ball shape (using hydrocarbon) was deposited on the top of MWCNT.*

*Diameter of MWCNT is 20, 60 and 120 nm. Scale bar is 1 μm.*

Using the FIB process, CNT can be welded and shorten to use for AFM scanning.

by tilting the sample back and forth by 52o.

*21st Century Surface Science - a Handbook*

**Figure 3.**

**262**

Herein, we have validated the working of the algorithm to measure the 3D profile of different samples (AAO and silicon pillars). Proposed algorithm enjoys all the advantages of conventional AFM. In addition to this, it has the capability to the measure the high aspect ratio features present above or below the sample. Here, we have used AAO sample to measure the internal morphology of the holes and 3D scanning of silicon pillars. To measure the internal morphology of the hole, firstly, the raster scan is performed using the MWCNTs based AFM probe: MWCNTs probe is attached using the FIB process on the Tungsten (W) apex which is further attached with the one prong of quartz tuning fork. Once raster scan is completed, feedback loop is turned off to start the 3D scanning of the hole. Positions of the holes and protrusion can be clearly seen from the raster scan image. Secondly, the algorithm is executed to scan the internal topography of via hole. Proposed algorithm can be summed up in four steps: Finding the location of hole(s), moving the tip to the bottom of the hole to be scanned internally, finding the first sidewall along the lateral direction, making the motion corresponding to the sidewall detected.

#### **3.1 Finding the location of holes**

Location of the hole can be founded in various ways. One of the easiest ways is do the raster scan over certain area at high resolution. However, due to the high aspect ratio of the AAO holes, the tip wear might occur which is not suitable to do the 3D scanning of the hole. For 3D scanning of the hole, the tip should have perfect symmetry so that the accurate imaging can be done inside of the hole. Other method is to do the force-distance curve or mapping, amplitude-distance curves or mapping over a certain area i.e. doing the amplitude-distance curve at each pixel and recording the coordinates and Z position. Lowest Z scanner value can be marked by red color and highest value with blue. Each individual pixel locally quantifies physical properties and interactions. By this, lot of information (elasticity, adhesion) can be mapped directly of the sample internal topography. In the **Figure 4**, we have done the mapping of certain area of AAO in which location of the can be clearly seen with different color contrast. In **Figure 4(i)**, FD curve is done on solid surface which has sharp change at 450 nm and on contrary side, **Figure 4(ii)** has rapid decrease in amplitude at 201 nm which shows which shows the tip can penetrate up to 249 nm. However, at this stage, tip is restricted not to go beyond the set point level. Later on, once the location of the hole is determined, tip can be pushed deep inside to measure the exact depth of the hole.

tip pierces in to the hole, amplitudes remains constant. However, if the tip is not at the center or non-uniform hole, then it might experience van der Waals interaction between MWCNT and sidewall of AAO sample as shown in **Figures 4(c)**—**(ii)** and **5**. Zongwei et al. [28] have used the DVD surface for force curve measurement at the pit edge as well as on plane surface and they have found that the ripple and wavelike wrinkles can be easily occurred if the FD measurement was done at the pit's edge. However, this van der Waals forces might restrict the tip to reach at the bottom. To resolve this, the FD is measured away from the sidewalls so that van der Waals are less likely to appear when the tip is at the center of the hole as shown in

*Measuring the Blind Holes: Three-Dimensional Imaging of through Silicon via Using High…*

*DOI: http://dx.doi.org/10.5772/intechopen.92739*

Once can interpret the interaction of MWCNT to the sidewall by observing the FD curve profile as shown in **Figure 5**. At some point, amplitude tends to decrease due to interaction of AFM probe with sidewalls. As the interaction dominates the setpoint, i.e., amplitude decreased more than the setpoint, tip is retracted. **Figure 4** shows a good method to find the exact location of the hole. However, to measure the actual depth of the hole, this is not the point where the tip has to stop. We try to push the tip inside of the hole by tracking the amplitude change and it has been found that when the amplitude drops more than 60%, then we can say that it might

be the bottom of the hole or in worst case the tip has been stuck to the hole. However, this problem can be overcome by measuring the FD curve in near surrounding of hole and recording the XY-coordinates with scanner value. Coordinate with the lowest scanner value might be bottom of the hole. After that, the scanner will move to that coordinate having minimum *Z* value corresponds to the minimum position of the hole. We have used the AAO sample with pore diameter 350 nm and different depth (300 nm, 500 nm, 1 μm with tolerance5%). FD curve is performed on different which gives the real depth of the sample. With 120 nm sample, depth of 1st AAO sample was measured 915 nm, 485 and 285 nm for second sample which corresponds to the vendor supplied values. In addition to this, we have done FD curves at the same position many times to check the reproducibility of the data as well as tip wear was less likely to found (Appendix). More than 10 times FD curves are performed and the difference between the first and final

*MWCNT interaction with sidewall of via holes. (a) Amplitude-displacement curve was performed on AAO sample with depth 300 nm with pore diameter 350 nm. In 300 nm sample, first drop in amplitude shows that the tip entered in to the hole and it might experience the sidewall or AD curve is done near the sidewall. As tip moves towards the bottom of the hole, at some point, rapid drop in amplitude occurs which shows the tip touched to the bottom. (b) FD curve on AAO sample with depth 1* μm *and pore diameter was 350 nm. The*

*depth was scanned approximately 930 nm. In both case, 120 nm MWCNT was used.*

**Figure 4(e)**.

**Figure 5***.*

**265**

#### **Figure 4.**

*Finding the location of holes by mapping. (a) FD mapping scheme in which the highest Z scanner value (surface or protrusion) is marked by blue color and lowest scanner value (hole) is marked by red color. (b) Using QTF, followed by MWCNT and tungsten probe, amplitude-displacement curve was measured at different position of AAO sample. Rapid decrease in amplitude (without wrinkle-top graph in green) shows the surface and amplitude-distance measurement with wrinkles shows the location of hole (lower graph in pink). Difference in both points (where the probe touches the surface and bottom) gives the depth of hole. (c) Mapping was done using 20 nm MWCNT on AAO surface over 3 micron area. From the graph (i) and (ii), it is clear that when Amplitude-displacement curve done on surface it does not have any hump while measured on hole, it shows some humps which corresponds to the Van der Waals interaction of probe with the inner sidewall of hole. CNT goes 935 nm gives the depth of the hole (d) Line 1 and line 2 shows the hole profile or depth profile of hole. From this data depth of the hole can be estimated which is 480 nm (Sample AAO – Actual depth—500 nm as purchased). (e) FD curve taken at surface, at the centre of hole, near the side wall of AAO hole.*

#### **3.2 Finding the bottom and depth measurement of via hole**

First, for scanning the internal topography of the hole, one can click at the center of the hole and scanner will move at the center of the hole. Algorithm is designed in such a way that mouse click will bring the scanner to the required coordinates (at the center of the hole). Then the tip will move along *Z* axis to reach to the bottom of the hole. If the hole is not uniform, then the possibility still exist that the tip might not touch to the bottom due to the many local minima present inside. As the

*Measuring the Blind Holes: Three-Dimensional Imaging of through Silicon via Using High… DOI: http://dx.doi.org/10.5772/intechopen.92739*

tip pierces in to the hole, amplitudes remains constant. However, if the tip is not at the center or non-uniform hole, then it might experience van der Waals interaction between MWCNT and sidewall of AAO sample as shown in **Figures 4(c)**—**(ii)** and **5**. Zongwei et al. [28] have used the DVD surface for force curve measurement at the pit edge as well as on plane surface and they have found that the ripple and wavelike wrinkles can be easily occurred if the FD measurement was done at the pit's edge. However, this van der Waals forces might restrict the tip to reach at the bottom. To resolve this, the FD is measured away from the sidewalls so that van der Waals are less likely to appear when the tip is at the center of the hole as shown in **Figure 4(e)**.

Once can interpret the interaction of MWCNT to the sidewall by observing the FD curve profile as shown in **Figure 5**. At some point, amplitude tends to decrease due to interaction of AFM probe with sidewalls. As the interaction dominates the setpoint, i.e., amplitude decreased more than the setpoint, tip is retracted. **Figure 4** shows a good method to find the exact location of the hole. However, to measure the actual depth of the hole, this is not the point where the tip has to stop. We try to push the tip inside of the hole by tracking the amplitude change and it has been found that when the amplitude drops more than 60%, then we can say that it might be the bottom of the hole or in worst case the tip has been stuck to the hole. However, this problem can be overcome by measuring the FD curve in near surrounding of hole and recording the XY-coordinates with scanner value. Coordinate with the lowest scanner value might be bottom of the hole. After that, the scanner will move to that coordinate having minimum *Z* value corresponds to the minimum position of the hole. We have used the AAO sample with pore diameter 350 nm and different depth (300 nm, 500 nm, 1 μm with tolerance5%). FD curve is performed on different which gives the real depth of the sample. With 120 nm sample, depth of 1st AAO sample was measured 915 nm, 485 and 285 nm for second sample which corresponds to the vendor supplied values. In addition to this, we have done FD curves at the same position many times to check the reproducibility of the data as well as tip wear was less likely to found (Appendix). More than 10 times FD curves are performed and the difference between the first and final

#### **Figure 5***.*

**3.2 Finding the bottom and depth measurement of via hole**

**Figure 4.**

*21st Century Surface Science - a Handbook*

**264**

First, for scanning the internal topography of the hole, one can click at the center of the hole and scanner will move at the center of the hole. Algorithm is designed in such a way that mouse click will bring the scanner to the required coordinates (at the center of the hole). Then the tip will move along *Z* axis to reach to the bottom of the hole. If the hole is not uniform, then the possibility still exist that the tip might not touch to the bottom due to the many local minima present inside. As the

*Finding the location of holes by mapping. (a) FD mapping scheme in which the highest Z scanner value (surface or protrusion) is marked by blue color and lowest scanner value (hole) is marked by red color. (b) Using QTF, followed by MWCNT and tungsten probe, amplitude-displacement curve was measured at different position of AAO sample. Rapid decrease in amplitude (without wrinkle-top graph in green) shows the surface and amplitude-distance measurement with wrinkles shows the location of hole (lower graph in pink). Difference in both points (where the probe touches the surface and bottom) gives the depth of hole. (c) Mapping was done using 20 nm MWCNT on AAO surface over 3 micron area. From the graph (i) and (ii), it is clear that when Amplitude-displacement curve done on surface it does not have any hump while measured on hole, it shows some humps which corresponds to the Van der Waals interaction of probe with the inner sidewall of hole. CNT goes 935 nm gives the depth of the hole (d) Line 1 and line 2 shows the hole profile or depth profile of hole. From this data depth of the hole can be estimated which is 480 nm (Sample AAO – Actual depth—500 nm as purchased). (e) FD curve taken at surface, at the centre of hole, near the side wall of AAO hole.*

*MWCNT interaction with sidewall of via holes. (a) Amplitude-displacement curve was performed on AAO sample with depth 300 nm with pore diameter 350 nm. In 300 nm sample, first drop in amplitude shows that the tip entered in to the hole and it might experience the sidewall or AD curve is done near the sidewall. As tip moves towards the bottom of the hole, at some point, rapid drop in amplitude occurs which shows the tip touched to the bottom. (b) FD curve on AAO sample with depth 1* μm *and pore diameter was 350 nm. The depth was scanned approximately 930 nm. In both case, 120 nm MWCNT was used.*

depth was only 10 nm. However, the Van der Waals forces may vary measurement-to-measurement as the tip goes deep inside the hole. Moreover, there was no significant damage to the apex of the tip. In **Figure 5(c)**, 120 nm shows the best result in stability as well as depth measurement and 3D scanning of deep holes. On the contrary side, 20 nm is suitable for 3D scanning of shallow holes as well as protrusion.

For example, tip move one step in +X direction and move back by tracking the amplitude. If the amplitude remains the same, it means there is no sidewall along +X direction. Similarly, the same phenomena is repeated by scanner for other axes (+Y, X, Y). The direction of sidewall is checked at every step. If the sidewall is detected in any direction, the corresponding motion will be performed by the

*Measuring the Blind Holes: Three-Dimensional Imaging of through Silicon via Using High…*

As the tip is scanning to find the sidewall along all the direction, at some point, the sidewall will be detected along +X axis and according the algorithm the tip will move along +Y direction. After that, the direction of sidewall will be checked again. If the sidewall is along +Y direction, then the motion will be along -X axis. Similarly, following the counterclockwise scheme, motion will be along +X direction if the sidewall will be in -X direction. If at next step, tip found no sidewall then the scanner will move in the direction where the last sidewall was. This technique helps the tip to strictly follow the boundary of the hole and with the motion decided by the algorithm, the perfect boundary of the feature can be determined sophisticatedly. The situation is more interesting when the tip experience two or three sidewalls and motion of the scanner will be followed as described in **Table A1** (Appendix). However, if all sidewall direction is found then tip is assumed to be trapped and should be retracted along +Z direction for its safety. **Figure 6** shows the working strategy to find the side wall of algorithm to track the boundary of the hole and tracking the amplitude as shown in **Figure 7**. To know the exact shape of the boundary, X and Y step size can be varied. However, there are two types of steps, i.e., jump step and motion step. The first one is jump step along any axis to check the direction of sidewall, which is usually greater than the motion step size. This is necessary to make the tip at a safe distance so that the tip not wear or to minimize the van der Waals forces between the tip and the sidewall. The motion step can be decreased to few nanometers for high resolution and to measure the exact shape of feature. After that, arithmetic is done by the algorithm, by considering the jump step, to calculate the diameter of each complete rim scan by adding the jump step to

*Working strategy of Algorithm to find the sidewall and scan the via holes. (a) Algorithm record the amplitude before and after moving towards the position of sidewall. If there is sidewall, rapid decrease in amplitude is detected as can be seen in (b). (c) Motion of the tip to follow the boundary of the via hole. The step size can be varied for high resolution. Tip start from the centre of the hole and start moving along +X direction to find sidewall. As it finds the sidewall, it starts following the boundary of hole in xy plane in counter clockwise.*

**3.4 Making the motion corresponding to the sidewall detected**

scanner (Appendix).

*DOI: http://dx.doi.org/10.5772/intechopen.92739*

diameter obtained from 3D scanning.

**Figure 7.**

**267**
