Atomair traceerbaar Nanostructuur Fabrication

The overall goal of the following experiment is to fabricate silicon nano structures with traceability to the atomic lattice using direct metal oxide etch mask growth and reactive ion etching. The ultimate precision of this procedure involves removing precise areas of a hydrogen passivation layer on a silicone chip using a scanning tunneling microscope tip as a second step, the pattern surface is exposed using an atomic layer deposition process that selectively deposits titanium dioxide and acts as a mask against reactive ion etching. Next reactive ion etching is performed in order to remove silicon from the surface in all areas except those that had been patterned previously.

The results show the ability to fabricate up to 20 nanometer tall structures with critical dimensions well below 10 nanometers. The main advantage of this technique over more conventional methods like e-beam or optical lithography, is that the initial metrology steps in the STM provide atomic scale information. This method can help answer key questions in nanotechnology, such as what are the precise interactions between nanostructures placed at very well-defined positions relative to each other?

Generally individuals new to this method will struggle because there are so many steps and opportunities to damage the sample. So we first thought of this method when we were trying to maximize the thickness of silicon dioxide etch masks, which we were writing on silicon, using an A FM and an STM tip in an oxidizing atmosphere. Instead, by combining hydrogen lithography with atomic layer deposition, we were able to obtain a similar aerial control while actually gaining a greater degree of freedom in the growth direction.

Visual demonstration of this method is critical as the transfer and pattern location steps are difficult to learn because each individual must perform their own steps correctly and be able to understand position location instructions. To begin, prepare and mount a silicon 1 0 0 chip with fiducial marks in the sample holder of a scanning tunneling microscope, and perform a flash cycle and passivation as described in the accompanying text protocol. Next, transfer the sample into the scanning tunneling microscope and bring the sample and tip into tunneling range.

Use a camera with a resolving power of better than 20 micron spot size to take a high resolution optical image of a tip sample junction des skew and resize the optical image so that it represents an undistorted reproduction of the fiducial marks with the tip location observed. Next, design the HDL patterns to be produced, including both experimental patterns and serpentine identification patterns. Fracture the overall patterns into fundamental shapes in order to define the basic vectors that will be followed by the tip.

When applying the AP mode and FE mode HDL conditions, then use lattice information from the silicon surface. To determine the ultimate tip path, use atomically precise HDL, also known as AP mode lithography for small areas or those areas requiring atomic precision edges using the vector outputs from the previous step. Perform HDL using field emission mode lithography for large areas with a sample bias of seven to nine volts, a current of one nano amp and 0.2 milli klos per centimeter.

Next, perform scanning tunneling microscope metrology on the desired HDL patterned areas by imaging with a minus 2.25 volt sample bias and a 0.2 nano amp tunneling current. Then disengage the tip from the sample and move the sample back to the load lock. Protect the sample by contacting it with an inert flat substrate such as clean sapphire once protected, close the valves to any pumps, and then introduce nitrogen gas to the chamber as quickly as possible.

When the chamber is vented, remove the sample from the system. See here a closeup of the sample shielding assembly using polytetrafluoroethylene or titanium tweezers. Quickly move the sample to the transporter, keeping the front side of the sample protected.

Install the cover over the sample and loosely assemble the pressurized sample transporter. Flush the transporter with ultrapure argonne for one minute, and then seal the sample transporter with a small positive pressure of Argonne on. Perform these steps to protect the sample in between each step in the process in this condition.

The sample will remain stable for up to one month. Preheat the atomic layer deposition chamber to 100 degrees Celsius. Then open the sample transporter and use stainless steel free tweezers to quickly transfer the to the deposition chamber.

Making note of the sample and control chip's position and orientation. Close the chamber and purge it using a flow of argon and a pressure of less than 0.2 millibars for one hour. Then perform 80 repeated cycles of atomic layer deposition to grow a 2.8 nanometer thick layer of amorphous titania on the sample using the recipe described in the accompanying text protocol.

Once complete, quickly move the sample back into the transporter and purge with Argonne. After removing the sample from the transporter securely, install it into the A FM system using a mechanical mounting method such as a clamping system or vacuum chuck. Focus the A FM camera onto the sample and locate the fiducial markings on the sample surface to align the A FM tip to the area where nano patterns are expected to be found.

Using the height and phase information at the highest resolution, scan the sample until the locator pattern regions are identified. Then take an image of the desired regions using the highest image quality and resolution available. Once the area of interest has been imaged, remove the sample and place it back into the transporter under argon gas.

While preparing for reactive ion etching, chill the capacitive coupled reactive ion etcher reactor to minus 110 degrees Celsius. Then remove the sample from the transporter and load the sample and any control chips into its induction chamber. Using conductive paste and pump the chamber down to 7.5 times 10 to the minus six millibars.

Stabilize the system for three minutes, then flow oxygen at eight standard cubic centimeters per minute. The argon at 40 standard cubic centimeters per minute and the sulfur hexa fluoride at 20 standard cubic centimeters per minute. Strike plasma using a 150 watt RF discharge.

Then modify the gas flow and etch for one minute using flow rates of 52 standard cubic centimeters per minute for sulfur hexa fluoride, eight standards cubic centimeters per minute for oxygen following reactive ion etching. Place the sample back into the transporter under argon gas. Open the sample transporter and securely install the sample, the SEM mount.

Then introduce the sample assembly into the SEM, pump down the chamber, and then locate and focus on the fiducial markers. Adjust the working distance as necessary and optimize the focus, the brightness and the contrast to minimize carbon deposition on the patterns. Optimize the focus using nearby non-essential features.

Once optimized, identify the approximate pattern location on the sample. Then move to the patterns and acquire plan view images and measurements. Then perform a typical SEM system closing routine and dismount the sample as prescribed by the SEM manufacturer.

Secure the sample back into the transporter under argon. At this point, the samples are robust and can be stored for an indefinite period of time. Shown here are representative scanning tunneling microscope images of HDL patterns created using AP mode only.

A combination of AP and field emission modes where AP mode was used to write each edge and field emission mode alone in order to achieve the best mask production. Using AP HDL patterns, a high degree of selectivity must be possible using atomic force microscopy. The height of the titanium oxide deposited on the pattern regions was compared to deposition on background regions.

This sample showed an incubation of about 20 cycles for the tallest background growth. Here, two serpentine patterns are written on a pitch of 10 nanometers using FE mode HDL. By rotating the patterns 90 degrees relative to each other, a grid is created.

This same pattern is shown here using a FM following the mask deposition of 2.8 nanometers of titanium oxide. Due to tip convolution effects the openings in the pattern are difficult to resolve. After reactive ion etching, approximately 60%of the desired openings were transferred into the substrate, indicating that this pattern size and density is approximately the limit for effective nanostructure fabrication using only FE mode HDL Once mastered.

This technique can be done in approximately three days if performed properly with most of the time devoted to ultrahigh, vacuum sample preparation and transport between locations if necessary. While attempting this procedure, it's important to keep the samples clean and to protect the background ation After this procedure. Other techniques like nano imprint lithography can be used to scale up the nano fabrication production capabilities of this technique.

After watching this video, you should have a good understanding of how to carefully handle samples to fabricate single nanometer scale structures. Precautions, such as gas dilution should always be used when performing this process. Otherwise, damage to the a LD pumping systems can be caused.