The overall goal of the following experiment is to demonstrate shallow doping of silicon waivers and nanowires by using the monolayer contact doping method. This is achieved by reacting a donor substrate with a precursor solution for the formation of a self-assembled monolayer. The monolayer contains potential dopant atoms and serves as a well-defined and limited dopant source.
As a second step, the donor substrate is brought into contact with the target substrate, and both are an kneeled in a rapid thermal anal system. During the anal process, the monolayer molecules decompose and the doin to atoms are diffused and activated onto both the donor and target substrates. Next, the two substrates are separated and a four point probe setup is used for measuring sheet resistance.
Results show typical decrease in sheet resistance values measured for the target substrates, a kneeled for various ane times and temperatures. The main advantage of this technique over existing methods like ion implantation, is that it enables simple, yet reliable formation of very shallow doping profiles. This method is useful for surface doping of all wafers as well as nanostructures.
It can also be used to other systems such as 3D device architectures and more. Identify the silicon and the silicone with dielectric substrates that will be used as target and donor prepare to work with piranha solutions. Acquire enough acidic piranha solution of hydrogen peroxide and sulfuric acid to cover the substrates here about 120 milliliters.
Use extreme caution while working with the piranha solution. Place the substrates in an appropriate holder. Insert the holder into the piranha solution for 15 minutes.
After 15 minutes, retrieve the holder with the samples and rinse everything in deionized water three times. Next, submerge the holder and substrates in about 140 milliliters of a base perha solution of ammonium hydroxide, hydrogen peroxide and deionized water. Put this in an ultrasonic bath at 60 degrees Celsius for eight minutes After the bath, rinse the samples in deionized water three times, followed by a rinse in ethanol.
Then blow dry under a stream of nitrogen. Dry the samples in an oven at 115 degrees Celsius for 10 minutes. For monolayer formation, prepare a 1%volume concentration of tetraethyl methylene diphosphate in meline in a pressure safe vial.
There should be enough to cover the donor substrates about 20 milliliters. In this case, use tweezers to transfer the donor substrates to the vial and ensure they are covered. Then seal the vial.
Use a mineral oil bath heated to 100 degrees Celsius to heat the vial for two hours. Then let the vial cool for three minutes. Open the vial and rinse the samples three times in meline and three times in di chloro methane.
Then blow dry them under a nitrogen stream. Nanowire synthesis starts with cleaning a microscope glass. Slide in oxygen plasma for two minutes.
Apply a few drops of poly L Lycine solution on the slide to fully cover it. Wait for five minutes, then rinse with the ionized water and blow dry with nitrogen. Now apply a few drops of gold colloid solution to the slide to fully cover it.
Wait two minutes. Rinse with the ionized water and blow dry with nitrogen. Remove organic contaminants by using oxygen plasma for 30 seconds.
Proceed by inserting the glass slide with gold nanoparticles into a chemical vapor deposition chamber and evacuating it with evacuation. Complete Pre equilibrate the chamber at 440 degrees Celsius and 35 tor with a flow of 50 standard cubic centimeters per second of molecular hydrogen. Then start the deposition process by flowing two standard cubic centimeters per second of saline to obtain approximately 50 micrometer nanowires.
Perform the deposition for 30 minutes. Once completed, measure the nanowires via microscopy. After measuring the wires, place the slide with the Nanowire film.
In a vial, add ethanol to cover the slide by about one centimeter. Sonicate the vial for three seconds during which the solution should become slightly turbid. Preheat a hot plate to 150 degrees Celsius.
Then place upon it a silicon nitride silicon dioxide, silicone substrate. Add a few drops of nanowire suspension onto the heated surface. Once the liquid is dry, check the density of the nanowires on the surface using an optical microscope with a dark field filter.
In order to achieve a high yield of single nanowire devices, this drop cast process should produce surface densities of about 100 nanowires per square millimeter with a minimal nanowire length of about 20 micrometers. To achieve doping, the substrate with nanowires and the donor need to be put in contact. Place the target substrate in the rapid thermal anal chamber.
Put the donor substrate on the target substrate with the front side facing the target substrate. Start the anal process. These samples will be subjected to a six second ramp.
Time from room temperature to an anal temperature of 1000 degrees Celsius held for 40 seconds. Once the monolayer contact doped sample is made, prepare to measure its resistance. Obtain a 1%hydrofluoric acid solution and place the sample in it for five minutes.
To remove the oxide layer, rinse with the ionized water. Then isopropanol and blow dry with nitrogen. Measure the sheet resistance using a four point probe technique.
The typical current applied is one to 10 microamps. Common photolithography techniques are used to produce nanowire devices schematically. Start with the sample containing nanowires.
Heat the sample, then spin. Coat it with photo resist. After setting the resist, use a mask aligner to expose the electrode pattern and develop the samples.
Rinse and dry the samples. Then use an e-beam evaporator to evaporate nickel onto the surfaces. Finally, perform liftoff to reveal the devices to locate successfully formed nanowire field effect transistor devices and register their locations.
Use an optical microscope with a dark field filter. A semiconductor device analyzer and probe station setup is used to measure the IV curves for the selected devices. Connect a conductive stage to the analyzer as the gate terminal.
Then place the sample on it. Connect the probe needles as source and drain terminals to the analyzer. Then use the microscope camera of the probe station to bring the probe needles near the device and gently touch the electrodes with the needles.
Proceed with measurements. Treating intrinsic silicon wafers with phosphorus monolayer contact doping results in a monotonic decrease in the sheet Resistance values as a function of ane times longer ane times lead to higher doping levels as do higher anele temperatures. Lower sheet resistance values are correlated with activated doping concentration with lower sheet resistance indicating higher doping levels.
Further increase in the annal time will not result in further decrease of the sheet resistance since the docent dose is limited. This diagram shows IV curves of nanowire field effect transistors. The black curve is for an intrinsic device.
Compare this to the blue curve for the moderately doped monolayer. Contact dope FET in blue. It was a kneeled at 900 degrees Celsius for seconds.
The red curve is for a highly dope device, a kneeled at 1005 degrees Celsius for 10 seconds. Following this procedure, other methods like time of flight, secondary ion mass spectroscopy can be performed in order to further characterize properties like doping concentration profiles. After watching this video, you should have a good understanding of how to use the mono contact doping procedure for making ultra shallow doping profiles on wafers and nano wires.
