Shelender Kumar

Plasma etcher from kitchen Microwave Oven

I am trying to convert a kitchen microwave into a plasma etching device, and I have made a pumping stand from microwave oven although it does not look good, it works just fine. I am using a flat Aluminium piece with Teflon O ring, and I am using the top part of the 200 mm diameter of the desiccator for the sample space.

I created a good vacuum and then I filled with the desiccator with Ar gas but somehow plasma not striking. My doubt is on our very old kitchen microwave. I will try to make this work. Here is the picture of mine setup.

Kitchen microwave converted into plasma etcher

Carbon thermometer for very low temperature

Measuring low temperature is an art which has fascinated me a lot. It is a such a big deal that few companies are making good money out of it, e.g. Lakeshore cryogenics. Please do not me get it wrong but it is an art very few people know about it. Making perfect resistance bridge and avoiding grounding loops and avoiding EMI interference is a big deal, i.e. Lakeshore has the big chunk of the pie. Their resistances bridges are very famous. I have seen all the big companies (Oxford Instruments, Janis, Leiden cryogenics) using it for low-temperature measurements. Although there is one more company which I know makes resistance bridges is Picowatt(http://www.picowatt.fi/index1.html). There are various scale to calibrate the equipment at low temperature. They have been given different names:

Leiden Scale: The Leiden scale (°L) was used to calibrate low-temperature indirect measurements of Helium vapour pressure.

International temperature scale of 1990:

ITS-90 is an approximation of the thermodynamic temperature scale that facilitates the comparability and compatibility of temperature measurements internationally. The lowest temperature covered by ITS-90 is 0.65 K.

Provisional Low-temperature scale of 2000: PLTS 2000 is an equipment calibration standard for making measurements of very low temperatures, in the range of 0.9 mK to 1 K.

Suppose you notice that you have very stiff requirements for measuring low temperatures. Typically, measuring the temperature from room temperature to 1K people uses Cernox thin film, and below that, up to 50 mK, people use RuO2 thin film. Now each Cernox has to be calibrated against some standard sensor, while RuO2 behaves very well so if you calibrate one of the chips from a batch of many resistors, they will behave more or less same. Each calibrated thermometer can cost you around 6000$. To measure lower temperature than these scientists are using flux noise thermometer (http://www.magnicon.com), Co-60 crystals.

This prompted us to make cheap thermometers, this publication by Gábor A. Csáthy et al. (https://link.springer.com/article/10.1007/s10909-010-0192-5) showed us light to make cheap thermometers, although it seems very easy, it takes quite a lot of practice, myself have broken many thermometers. I did calibrate a thermometer, but it is not very good, but it gave me confidence that we will go through. I think reproducible was bad; it can be because I did not thermal recycled in liquid Helium to get thermometers' stability. In the below images, I will show some of the thermometer's machined body made by me, although it has been inspired by the author of the above paper and Lakeshore cryogenics. I will show a typical curve of the Carbon thermometer up to 50 mK.

Various type of casing for thermometer mounting

Calibration data for Carbon thermometer in log log scale

Liquid Helium level meter

We have a He3 refrigerator from the Janis cryogenics, but we do not have commercial liquid helium level meter as we had a bit of fund crunch. They cost as much as 1700 pounds https://www.iceoxford.com/Cryogenic-spares/Electronics/Level-Meters%20and%20Probes.htm.

To measure the liquid level, we use carbon composition 220-ohm resistors. Now their resistances were around 520 ohms at 4.2K, but the problem was that we were not sure about the liquid level as they were not very reliable cold vapour were making life very difficult. You can buy carbon composition resistors from ohmite .https://www.mouser.in/ProductDetail/Ohmite/OF221JE?qs=sGAEpiMZZMuDPtTs5Gda222dYFHSt95UKj4GiAXdzns%3D.

Another way of measuring liquid Helium level meter using 1/4 inch or 1/8 inch long SS tube with rubber band on one end. As soon as the tube touches the He liquid to He vapour phase frequency of the vibration of rubber band changes a lot.

But I wanted to make something electrical where some alarm or LED will glow up, so I found a very simple circuit diagram which works on the very simple principle that the resistor becomes high for 220-ohm composition resistor at 4.2 K, so the voltage drop increase and the voltage drop is just enough to glow the LED. here is the link of the paperhttps://aip.scitation.org/doi/pdf/10.1063/1.1142447

Low-temperature probe for LN2 measurements and 1/f noise measurements

You have to be very careful when you measure at low temperatures because it has been leak-tight otherwise everything will freeze. All electrical feed-thru has to be leak-tight, so I decided to make one. I made eight-pin leak-tight with Araldite and body of the probe from SS.

I also needed to make a breakout box which will house out BNC connector. The inner part of the probe is made from 1/2 inch solid SS cylinder, and for handling the sample, end part of the inner probe is made from copper.

I have used this probe for thermal cycling of my thermometers and 1/f noise measurements also.

Clapp/Colpitt oscillator circuit

One of my projects involves measuring the dissipation in superconductors or measuring the sliding friction of superconductors over a solid surface. This work was started by Jacquline Krim (https://www.physics.ncsu.edu/nanotribology/publications/ref75.pdf ). Although this work has created a lot of controversies, it still not disapprove thoroughly. They could see the change of dissipation and the frequency change while moving the sample inside and outside of liquid Helium, So we decided to make the crystal oscillator circuit used by the Krim. However, I found little success I must say that external circuitry to the crystal matters the most. Our cryo-stat and wires make the cable around 4 m of the length, and this is why we cannot measure the signal from the crystal.

I tried to make the Clapp oscillator circuit which looks like this.

Evaporation at water without heating

I think everyone of us know that water boils at low temperature at high altitudes but I am afraid of heights so I cannot try this experiment probably but fortunately we have vacuum pumps to do this job. Now we have sputtering chamber and so happens that one of the gun gone short so we got it repaired that means the chamber was in air for long time (bevel me it is very bad habit to keep the chamber in air but sometimes you cannot avoid it.) so when we started pumping you can see the vapors of ]moisture from the glass window, too make it more clear I will post a video here https://www.youtube.com/watch?v=B8gvuMi93Hw&feature=youtu.be ans I am also posting the boiling point of water at various pressure (https://www.engineeringtoolbox.com/water-evacuation-pressure-temperature-d_1686.html).

31st january 2021

The article which I wrote for the genral audience about my reserach work:

drive.google.com/file/d/1UsKLeTpxbRkmCGfb-ywJlQALCNBSDIMY/view?usp=sharing