A working DIY quartz tuning fork AFM

說明

This is a cost optimised quartz tuning fork (QFT) atomic force microscope (AFM). It is currently a working progress as I am trying to make it as accessible as possible through cost optimisation and making sure everything can be done DIY including the nanometer sharp tips required. The project is not yet very open-source friendly yet as I am using commercial AFM tips and am still working on making my own by electroetching some tungsten wire.

This cost optimisation has been achieved through a custom PCB with an stm32-based digital lock-in amplifier to demodulate the signal coming from the tuning fork at 32kHz. Its performance appears to be on par with the thousands worth of equipment we have in my lab that do the same thing. I am able to easily see 10s of nanometer features with the current 3D-printed setup but I am still optimising to perhaps get to atomic layer resolution.

I am hoping to eventually turn this into a kit for schools, university lab experiments and hobbyists.

細節

This is a fairly complex project as it stands specially from the embedded system point of view. I would like to break this down for now into 3 main components: the tip, the custom PCB and the stage.

- The tip is currently a commercial AFM tip I was able to get for free from my lab to test things out. But I am working on an etching jig to make my own tips out of watch tuning forks and electro etched tungsten wire similar to the Mad City Labs AFM https://www.youtube.com/watch?v=K_k5UsHlwN4

- The custom PCB was fully designed by me and is aggressively cost optimised and will be further cost optimised again to reduce the cost further. The fully assembled price per PCB stands at something like 70ish bucks from JLCPCB. It's still a working progress as there are some things to optimise but as it stands it works fine. Software is really the magic here. I implemented a digital lock-in amplifier with its own DDS signal generator as well as PID for amplitude modulated (AM) and PLL frequency modulated (FM) AFM. Additionally, there is also Q control to dampen the tuning fork if someone wants to operate in AM mode in vacuum. The lock-in is implemented using the STM's own included peripheral ADC and DAC making this extremely cheap since the uC chip costs like 7 bucks. I actually have more than one lock-in channel as I might add closed loop control to the stage for ultimate precision.

-The stage is a fun one. For the testing stage in the pictures, I have a 3D printed flexure stage. There are 2 linear 3D printed flexure stages stacked on top of each other forming the x and y axis. The z axis is a piezo buzzer. The flexure stage is based on the work of Eric Clot who realised hepatic feedback piezos are great cheap precision actuators! His project is linked here https://hackaday.io/project/202424-haptic-piezo-for-high-precision-actuation I want to design a flexure stage and have this CNC machined by JLCPCB (Wire EDM is very expensive...). There are currently some piezo hysteresis issues I am trying to sort out (lines outside the calibration grid squares in the images) but I am looking into this so this can be close to a commercial educational system for <1/20th of the price tag.

I will try my best to document the project in detail but I am also currently doing a Physics PhD so it might take some time to get updates on here.

Project Logs

折疊

Current state of the project
Luis Mestre • 9 hours ago • 0 comments

Current state of the project is that it is working and rather well. I actually had a look on hackaday and there seem to be other quartz tuning fork AFMs going but no one seems to have gotten nice images. I am fortunate enough to be in the right scientific environment for my work where I can ask people questions when I get stuck and therefore I wanted to put this project on here to perhaps show the other projects is doable and for them to take some ideas out of this was well. I am not sure how much time I will have to document this very throughly especially because I am still improving the setup and want to do this once it's in its final state but I hope it's useful to someone. I want to make a little kit people can buy and just assemble for university labs and hobbyists but I will also work on 3D printed stages anyone can build at home with some time which should be dirt cheap and these I will try to put on here in more detail. The PCB etc I will document at the end once I am happy with it's final state as this was my first iteration and still needs improving.

Now for the cool stuff. The images shown are from AFM calibration grids one has steps of 20nm which really require a decent AFM to see. I also tried to image a graphene device with some wires coming out. Bigger calibration grid is quite easy to see. Some examples of images are below

This is the image below

as you can see the forwards and backwards scans during the raster scan have different sizes (the backwards scan is stretched in comparison with the forwards one) I believe this is the piezo hysteresis in the stage. This is fixed by scanning in a single direction. That is, once you reach the end of the forwards scan, you lift the tip move it back reengage the PID and scan again, this unfortunately adds some time to the scan but fixes this issue more or less.

New images I took are now with improved noise from shielding on the tuning fork wires leading to the transimpedance amplifier (TIA) and below you can see a 20nm step from the calibration grid with 20nm high 1.5um wide squares

Here is an example of the 20nm step with improved noise. The previous steps looked like this:

With the improved shielding on the tuning fork wires to the TIA the same 20nm step is much cleaner. I believe now I am limited by environmental vibrations due to my damping being a bike inner tube, to see atomic layers, I likely need and enclosure and the setup suspended by springs with eddy current damping from magnets similar to Dan Berard's STM. but I would say this is pretty good already for most applications like looking at cells or DNA.

I also imaged the connections wires to a graphene device kindly given to me by a friend where you can see 10s of nm high wires, It's a little hard to see in the topography image without better processing in gwyddion but you can see the e-beam evaporated gold wires coming out from the graphene flake in the amplitude scan quite well.

below is the setup the device is in the middle of the wires

Here is the optical microscope image

I believe this image is on one of the edges of the flake, as I didn't have a great microscope to check where I was and the tip is moved with a hand micrometer stage. With only 15x15um scanning range and the images taking a while it's hard to hit the actual 10ish um device (I didn't have too much time that evening). However, it is still possible to see the 10nm high wires and the graphene should be quite thin as well.

These are all half images because I was trying quite a few things out and I am limited by the bandwidth of my loaded Z piezo buzzer. This means I have to scan quite slowly and the images with high resolution can take quite a bit of time compared to a standard cantilever AFM that will take like 3 min. I actually compared my images to the system we have in the building costing...
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