STM Heating Automation

1. STM Heating Automation ECE 345 Senior Design Project Aditya Vaidya and Kanishk Kejriwal

2. Scanning Tunneling Microscope Basics • STM is used to scan and generate images of nanostructures • It is also a tool for Nanofabrication • Scanning is usually done at extreme temperatures to ensure absence of redundant microscopic particle

3. STM - How does it work? • As the tip scans the surface laterally, changes in the shape of the sample are registered as current changes. • Images are generated as piezoelectric motors adjust the height of the tip and maintain an average gap current. http://dpmc.unige.ch/gr_fischer/localprobe.html

4. Sample Heating • Samples have to be heated to temperatures as high as 1100°C to drive off impurities. • Pressure in the UHV chamber has to be maintained below 1E-9 Torr while heating. • Need to ensure that the pressure and temperature are regulated.

5. Why Automate? • Someone has to keep checking the parameters. Inaccurate sometimes. • Need a mechanism that regulates the physical parameters while heating.

6. Proposed Solution • Save time by sending commands in the form of data to power supply. • Labview communicates with devices. • Serial/GPIB communications with devices.

7. Individual Devices and how they communicate • Varian Multiguage (RS232/Serial) • Sorensen/Agilent Power Supply (GPIB/IEEE488.2) • Minolta Cyclops 52 Pyrometer (RS232/Serial)

8. Block Diagram of proposed solution Pyrometer Serial/RS232 Multiguage Lab View Power Supply

9. Specifications • Aim: Maintain Si sample at high temperature without exceeding pressure limit. Between 800C – 1100C • The power supply plays an integral part • Pyrometer specification:Temperature Range: 650C – 2000CError Margin: 2% • Pressure Gauge: Pressure as low as: 1E-18 Error Margin: 5%

10. Specifications (cont.) • Error margin of Agilent E3631A 2% Error margin of program 5%Resulting error margin of 7.1% Fairly good error margin. • Response time of the program to changes in system:For temperature: 300msFor pressure: ~1s

11. Sample Specifications • Sample is usually Silicon • Boron doped, p-type • Resistivity 0.01 – 0.03 Ω cm • Total Resistance at room temperature is due to contacts with the holder (~40 Ω)

12. Advantages of Labview • User friendly interface • Communication with devices made easy • VISA (Virtual Instrument Software Architecture) • Drivers for Power supply available

13. Program Layout • There are 6 main parts of the program: -Main Controller -Power Initialize -Pressure Monitor -Temperature Monitor -Current Change Calculator -Power Controller • Highly modular design

14. Visa User Input Area Program Status Output User interface Easy to follow Not too many controls

15. VI Design • The Main VI communicates with all the devices using several sub-VIs • Two nested loops • Once the process has been run for required time, it shuts

16. Power Initialize VI • Uses the initial resistance of the sample to supply current. • The power output is so small initially that we can assume constant resistance.

17. Pressure Monitor VI • Following are the cases: Pressure between Desired and critical Pressure below desired Pressure above critical • Increase power only if pressure below desired • If pressure above critical, reduce power

18. Temperature Monitor VI • Compares temperature with upper and lower limit. • Power increased only if temperature and pressure are below the range • Power decreased if temperature is beyond range

19. Current Change VI • Algorithm: We can increase current only if pressure below desired value. We must decrease current if pressure above critical. Reduce current if temp. above desired range. • The change in current is 0.1A which is a suitable step size.

20. Power Regulator VI • Receives input from the Change calculator • Sets the new value in the supply

21. Testing Procedure • Power Initializer - Used a potentiometer and supplied a certain current to it. Slowly changed the resistance to observe behavior. • Pressure Gauge - Connected the pressure gauge to a chamber and studied the behavior as the pressure changed due to higher power.

22. Testing (cont.) • Pyrometer - The pyrometer broke during experimentation hence extenisive tests could not be executed. Tested the basic communication with the device. • Unit testing - Conducted heating experiment on a sample using the program. The program responded perfectly to changes in environment variables and the required temperature was reached.

23. Results from test • Change in resistance of sample holder • Temperature measurements • Change in pressure of chamber

24. Challenges • The hardest part of the project was communicating with the devices. • There were significant time delays as the communicated values were always erroneous. • Eventually used the Agilent supply from lab. • Pyrometer stopped working in the middle of the project. Has not been fixed yet.

25. Future improvements • If parameter data is needed for future reference, an excel worksheet could be added that would store data at regular intervals. • Control the Sorensen power supply with the Agilent using A/D scaling.

26. Acknowledgments • Professor Joe Lyding for giving us a chance to work on this project • Peter Albrecht and Matt Zettle for STM/Hardware orientation • Mark for helping us with the Power supply, the central device in this Project.