Electrostatic force microscope

Transcription

Moving the detector around, we can see a dot moving on our screen across the position sensitive photo diode. So we will zero out the detector by adjusting the knobs so that the simulated laser beam is right in the center of our position sensitive photo detector. We will use the software to adjust the illumination of our sample until our optical microscope is bright enough to see the image. Then we will use a track ball to focus on the tip. Next we will physically move the optical microscope until the crosshairs on our screen are centered on the backside of our tip. We will now place the sample on the vacuum chuck of the instrument. The silver paste is dry now and it goes from the conducting top surface of my sample around the edge and to the bottom so that when placed on the stainless steel chuck, it will make electrical contact to top of my sample. We then turn the vacuum chuck on and rotate the sample beneath the AFM tip making sure that the wire for the conducting tip connection to the amplifier does not touch the sample chuck because that will cause interference with the images. So having focused the microscope on the AFM tip, we are now going to focus the microscope on the actual surface of our sample. And this is how the microscope actually aligns the tip very near to the surface without actually ramming into it by accident. So we will start off by clicking on the focus surface icon. So in the focus surface mode, moving the track ball will move the sample mount and allow you to position it beneath your AFM tip. And then using the focus mode will actually bring the microscope head closer in to the surface. Usually I would observe by eye and get the tip as close to the surface as I reasonably can. And then I will look at the optical image on my screen. So we have two options when actually focusing on our surface. We can choose to focus on the surface itself if we expect it to be dusty or not reflective. However, if you have a clean transparent surface like we’re working with now, it’s easier to focus on the reflection of the tip on that surface so we will select focus on tip reflection. We will zoom in slightly on our chip which is right now very blurry. And then we will bring in our microscope head closer to our sample. We should see the tip reflection come into focus very nicely, like so. Now to actually select a spot on the sample that we would like to image we can switch back to the surface focus mode and the microscope will adjust itself such that you should be focused on the surface. We’re pretty close you can see here there is fleck on the surface so I might fine tune my focusing to bring that into good focus. I’ll adjust my illumination slightly and then using the trackball I’ll move around on my surface until I find a region that is of interest to me or just happens to be clean. When that’s done I will click out of this menu and I will set my software settings to be prepared to engage the tip with the sample. So the important settings are that you in the TUNA mode, that the z -limit is set at its maximum value. We’ll pick a deflection set point that is somewhat larger than the vertical deflection of our tip as it is zeroed out here so in this case our vertical deflection on the tip is minus .02 volts so as a starting deflection set point I’ll select positive .03 volts. And then avoid damage to our soft platinum tip on our hard metal oxide I will engage with the scan size set to zero so that the tip will come down and sit on a single point rather than coming down in scan. And I will also turn off the slow scan access. One last check to make sure everything is as it should be. Close the acoustic and electrical noise shielding, lock it, and then I can leave my static isolation bracelet here. So what I’m actually going to do is I’m going to engage the tip and it is going to come down and just sit at a point. Now the reason is that I do it this way is that I want to do a series of electrical measurements, and my tip is a very soft metal and it’s on a very hard abrasive metal oxide and so if I come down and start trying to image, I’m basically going to be rubbing the tip off and contaminating it. So I will do an array of those and then once I’ve run that array of current voltage curves I will back out then actually take the image. And I just set it at its now going to do this array of points, and so these are the current voltage curves and they just pop up like this. Usually I will spend a little bit time adjusting the settings until I get them to where I feel they are appropriate. I usually do an array that’s 25 by 25 points and I choose the spacing such that it comparable to the area that the AFM tip covers so my array ends up being 500 nanometers by 500 nanometers. I can just use the IV curves individually or I can go and offline later on fit the IV curves and determine the electrical properties which can then be mapped because I know where they were taken. So we’ll just scan 500 nanometers square, at one half hertz, and adjust my gains. I’m just adjusting my gains so to try and get what I consider the best overlap between the trace and the re-trace of the tip without introducing too much noise. I can set my microscope to start at the top of the image and then begin obtaining data. The left hand side represents topography or height, and the right hand side represents current, with in this case the black being more current and the reddish brown representing zero current.