The Simplest Homemade Atomic Force Microscope

History was made with the atomic force microscope. It allows individual atoms to be made visible. The AFM was invented by IBM scientists in 1985. The precursor to the AFM, the scanning tunneling microscope (STM), was developed by Gerd Binnig and Heinrich Rohrer in the early 1980s at IBM Research – Zurich, a development that earned them the 1986 Nobel Prize for Physics. Binnig invented the atomic force microscope and the first experimental implementation was made by Binnig, Quate and Gerber in 1986. The first commercially available atomic force microscope was introduced in 1989. The AFM is one of the foremost tools for imaging, measuring, and manipulating matter at the nanoscale.

An atomic force microscope (AFM) is used to visualize the smallest surface structures by moving a cantilever arm with a very thin, pointed tip on its underside over a sample to be examined. In contact mode, the tip touches the surface. The smallest atomic irregularities cause the cantilever arm to move accordingly. A mirror is located on the top of the cantilever arm, and a laser is directed at this mirror. If the cantilever arm moves slightly up or down, the laser beam is reflected slightly differently and hits a light sensor at a different location. As a result, the AFM adjusts the z-height of the object so that the laser beam hits the light sensor centrally again. The z-axis adjustment required for this is then a measure of the atomic forces between the tip and the surface, or rather, of the surface structure.

More information: Wikipedia

My setup differs slightly from the one described above. Instead of using feedback to a piezo to change the z-axis of the stage, I use a line sensor to detect the laser beam and leave the stage height unchanged. In my setup, a bend in the cantilever results in a different point of impact for the laser on the line sensor. The changed position of the line sensor is then a measure of the surface structure.

The parts you will need for this project:

1. Arduino Mega
2. 320 x 480 pixel display compatible with the Arduino Mega
3. line laser
4. TSL1401 line sensor from aliexpress
5. two old DVD drives
6. two step down modules with LM2596
7. stepper motor drive f.e. easydriver
8. thin aluminum sheet for the cantilever
9. a compass tip
10. small surface mirror
11. 10 kohms resistor
12. button
13. 12V power supply
14. a test object. f.e. a coin

To adjust the sample in the x and y directions, we'll need two old DVD drives. I've had good experiences with the NEC DV-5800B model, as its tray is very well suited for mounting an additional drive.

Furthermore, a threaded rod and a rectangular steel tube are required, which will later hold the cantilever. For the metal tip that scans the object under investigation, we use a standard compass point. This is glued, along with its mount, to a piece of aluminum sheet. We bend the sheet, which serves as the cantilever, slightly and glue a surface mirror to the top. This will later be illuminated by the line laser.

You'll also need a TSL1401 line sensor. I was able to purchase this sensor inexpensively on AliExpress. I've attached the pinout as an image. If the line laser is too powerful and overloads the sensor, you may need a neutral density filter to reduce the laser intensity. This neutral density filter is then simply glued over the line sensor.

The Arduino program always displays the current intensity curve before the scan. A narrow peak should appear approximately in the center of the sensor. Only then should you press the start button for the scan.

During my first attempts, I noticed that my compass tip was getting caught on edges and tilting to the side. To prevent this, I made an additional compass tip guide. This prevents the compass tip from getting caught, as the guide now only allows it to move up and down.

I've attached the complete circuit diagram. It requires a 12 V power supply for operation. If you use a 5V line laser you have to adjust the one step down module to +5V and you don't need the 20 ohms resistor. The resistor is only necessary when you take a f.e. 100 mA laser diode without driver.

I first used a coin as a test object. The resolution in the x- and y-direction is approximately 20 micrometers. In the z-direction, heights of less than 25 micrometers can be resolved. This allows, for example, the 35 µm thick copper layer of a perforated circuit board to be easily identified.

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