12 Feb 19. Images and text courtesy of John Bean, University of Virginia

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Ellen Goodwin
5 years ago
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Here we take the covers off the atomic force microscope.

The probe, itself, is under the cover at the right.

The probe is precisely shaped by a process called silicon micromachining.

crystallographic planes. This point is almost atomically sharp.

1 Here we take the covers off the atomic force microscope. Inside, there is a circuit board that controls and monitors the probe s movement. The probe, itself, is under the cover at the right. 1 2 The probe is held at the bottom of a small moveable tripod (more about this later). The probe is precisely shaped by a process called silicon micromachining. This process combines photographic patterning with chemical etching to shape silicon along precise crystallographic planes. This point is almost atomically sharp. The cantilever probe, the tripod, the base on which they are mounted, and the front cover, can all move up and down as the single "AFM head" unit shown here. The head is lifted and then lowered when a new experimental sample is loaded. It rides on two square ball bearing rails mounted vertically at each side of the head (shown here in gray 3 4 1

The mating vertical rails are attached to the body of the AFM. The head is then driven up and down by a motor and worm drive assembly.

. The coarse height of the AFM is adjusted by the three large vertical screws at the edges of the unit (not now shown).

As the head is driven up and down, a ribbon wire flexes to maintain the electrical connection to the head.

2 The mating vertical rails are attached to the body of the AFM. The head is then driven up and down by a motor and worm drive assembly. The motor drives gears that that turn a vertical screw. This screw threads in and out of a brass bushing.. The coarse height of the AFM is adjusted by the three large vertical screws at the edges of the unit (not now shown). But the motorized movement here (of only a few tenths of a millimeter) allows a final automatic approach to the sample that minimizes the chance of damaging the cantilever s tip 5 6 As the head is driven up and down, a ribbon wire flexes to maintain the electrical connection to the head. But the really fine scale movement of the cantilever is controlled from inside the head assembly To monitor the cantilever probe movement, two small microscope lenses are built into the orange cover. The right one looks down on the cantilever from above. The left one uses the small prism inside the cover to produce a side view. Two small white light emitting diodes are mounted on the cover s tiny circuit board to provide illumination

To scan across the sample s surface, we need horizontal movement in two perpendicular directions.

As the right and top legs are pulled toward the magnets, the tripod rotates around the twisting spring at the base of the left leg The tripod is suspended on three flat shaped springs.

To produce vertical (Z) movement, the top electromagnet is energized. The top leg of the tripod is pulled back.

3 To scan across the sample s surface, we need horizontal movement in two perpendicular directions. X movement is produced by energizing the right electromagnetic twice as strongly as the top electromagnet. As the right and top legs are pulled toward the magnets, the tripod rotates around the twisting spring at the base of the left leg The tripod is suspended on three flat shaped springs. Behind these springs are three iron cylinders, each spaced slightly in front of a small electromagnet. The tip of the cantilever is then moved in X, Y, and Z directions by tilting the tripod. To produce vertical (Z) movement, the top electromagnet is energized. The top leg of the tripod is pulled back. The tripod then rotates around the two bottom springs as they twist to accommodate the movement of the top leg. This vertical movement will be used to track the peaks and valleys of the experimental Images and text courtesy of John Bean, sample s University surface. of Virginia 9 10 Y horizontal movement is then generated by energizing the left and center electromagnets (the left twice as strongly as the center). The tripod then twists around the spring at the base of the right leg. Different combinations of electromagnets will produce different cantilever motions. For instance, here, when all three electromagnets are energized together, the tripod and cantilever move straight back. Then, as the left electromagnet is energized even more strongly and the right electromagnet is deenergized, the cantilever moves to the left

Let s simplify things by fading away all but the three most critical components: 1. a semiconductor laser at the upper right. 2. a photodetector, at the upper left. 3.

Upon command, the AFM will uses the vertical ball bearing tracks to lower the head until the probe tip contacts the sample. But how does it sense this contact?

4 Let s simplify things by fading away all but the three most critical components: 1. a semiconductor laser at the upper right. 2. a photodetector, at the upper left. 3. the all important cantilever assembly. The really important part of that assembly is the tiny diving board like cantilever, and the ultrasharp point projecting down from its end. Upon command, the AFM will uses the vertical ball bearing tracks to lower the head until the probe tip contacts the sample. But how does it sense this contact? Unlike the STM, it does not use electrical current flowing into the sample (indeed, an advantage of AFM is that it can measure insulating samples). Instead, it lowers the head until the cantilever bends just enough that the reflected laser beam becomes centered in the photodetector. From this point onward, the feedback circuits of the AFM will continue to move the head so that the laser is held at this position To measure the shape of the sample, the electromagnets are energized to pull the AFM head backward. However, this sample has a sloped ridge at its center. As the probe tip runs up the edge of this ridge, the cantilever is bent even more strongly. And as it bends, the reflected laser beam moves to the back of the photodetector. But now the electronic feedback loops kick in. By further energizing the top electromagnet, they rotate the invisible tripod (laser, cantilever and photodetector) upward. This continues until the cantilever straightens out enough to return the reflected laser to the center of the photodetector But what happens if the bumps and ridges on the sample have very steep slopes? For instance, what if we were measuring the steep edges of silicon dioxide stripes on the surface of an integrated circuit?

In this situation, the ultra sharp cantilever tip might not slide successfully up onto oxide stripe. Instead it might break off.

There would also be problems if we were measuring the shape of a soft material, such as a strand of DNA.

What is required is an alternate method of operating the instrument known as "Dynamic Force" AFM To explain "Dynamic Force" AFM, let s first repair the tip, shrink the cantilever, and move in closer.

5 In this situation, the ultra sharp cantilever tip might not slide successfully up onto oxide stripe. Instead it might break off. This ruins the instrument s near atomic resolution (requiring us to buy and install a new micro machined cantilever assembly!). There would also be problems if we were measuring the shape of a soft material, such as a strand of DNA. In this case the sharp tip might just pull its way right through the strand, giving us no information about its shape. What is required is an alternate method of operating the instrument known as "Dynamic Force" AFM To explain "Dynamic Force" AFM, let s first repair the tip, shrink the cantilever, and move in closer. We then make visible the black plastic tripod on which these components are mounted. Notice the tan cube? This is a "piezoelectric crystal" (or just "piezo"). Piezoelectric crystals have the unusual property that, when a voltage is applied across them, they can shrink and expand in certain directions By cycling the voltage across the piezo, it repeatedly stretches and contracts in the vertical direction. This cycling occurs quickly (e.g., 200 MHz) The cantilever assembly is mounted just below the vibrating piezo. So the piezo s vibration also causes the cantilever to swing up and down. The reflected laser beam (bouncing off the cantilever arm) then also oscillates backward and forward across the face of the photodetector. But with no sample in place, the reflected laser beam still hits the photodetector forward of the yellow arrow But things change as the sample is now reintroduced. At the bottom of each swing, the point of the cantilever strikes the sample, kicking it upward. This alters the position of the reflected laser beam on the photodetector, shifting its oscillations slightly towards the rear

6 But things change as the sample is now reintroduced. At the bottom of each swing, the point of the cantilever strikes the sample, kicking it upward. This alters the position of the reflected laser beam on the photodetector, shifting its oscillations slightly towards the rear The electronic feedback circuit is then set to try and hold the average postion of the laser beam at the center of the photodetector. But then (as in the static mode) when the cantilever moves up a bump on the sample surface, the laser will move even farther back on the photodetector. The feedback circuit then kicks in turning up the top electromagnet. This rotates the whole tripod assembly restoring the laser s average postion to the center of the photodetector. Video 24 The net result is that, as the cantilever moves across the sample, it will only tap the sample at the very bottom of its swings. For this reason this is sometimes called the "tapping mode." Tapping has the huge advantage of allowing the tip to climb up abrupt slopes without breaking off. But "tapping mode" can still damage samples (and cantilever points) and it turns out that there is an even better way of operating the AFM: The piezo sfrequency is first scanned. When it passes through the cantilever s "resonant frequency" the cantilever s bending is maximized (also maximizing the laser s movement). This frequency is carefully recorded along with the amplitude and phase of the laser s oscillation. When the sample is reintroduced, the tripod is then lowered only until there is a barely detectable shift in this resonance. This shift can occur BEFORE the cantilever tip actually touches the sample! For instance, as the swinging tip nears the sample it can interact via "Van der Waals attraction." Video Video 25 6

7 The feedback circuit is then set to lower the tripod ONLY until it senses this very first weak disturbance in the cantilever s resonance. The result is a "noncontact mode" of AFM operation where the tip never quite touches the sample (in a classic atom to atom sense) This mode is ideal for rough and/or easily damaged samples! That describes the operation of the easyscan Atomic Force Microscope, and its main modes of operation. Bear in mind, however, that to make things visible we have exaggerated the movements of the tripod and cantilever by a thousand times or more. In reality, you can only just barely see the largest of these movements through the magnifying lenses built into the head of the instrument. We also changed the color of the laser from its true, invisible, infrared color to a red color that we could see. Video Video 26 7

Atomic Force Microscope

Atomic Force Microscope Preparation Before our first appointment to use the microscope, please watch the video tutorials at https://www.afmworkshop.com/atomic-force-microscope-animated-tutorials/. The