Instillation and setup of a Silicon Drift Detector EDS system. A SDD is a modern detector for Energy-dispersive X-ray spectroscopy that mounts onto an Electron Microscope and allows the user to analyze the composition of a sample. When electrons from the SEM hit the sample, it emits secondary electrons (along with many other particles and energies) which are used by the SEM to produce the image you see on screen and X-rays which are detected by the SDD to produce EDS information. The X-rays have a specific energy which is characteristic of the material and equal to the energy difference between excited and ground state electron orbitals in the atom.
The SDD (30mm2 in my case) quantizes the energies of impinging X-rays and amplifies them to be read by a computer. As compared to traditional Si(Li) detectors, SDDs are faster, have higher resolution, and do not require Liquid Nitrogen cooling.
The detector is mounted to the chamber at a 35° angle which makes it most convenient for use at a WD of 25mm. When using the SEM at lower working distances, the SDD probe is retracted out of the chamber as to avoid collision with the sample stage. The detector is chilled to -30°C with Peltier modules.
The detector has a built in preamplifier. The output is fed to a pulse processor containing an FPGA which connects to a laptop via ethernet.
With the SDD, EDS spectra can be acquired in less than a few minutes at high beam currents.
(Click on image to enlarge)
Thanks to Pulsetor LLC, Rick Mott, Jeff Thompson, David Bono, and John Guerard for their extreme generosity and help with the detector.
An Inficon Transpector2 HPR RGA was purchased from eBay and refurbished. I will use it for leak checking and ion selection experiments. Thanks to Aota Vac and Inficon for software assistance!
Put simply, an RGA is a small mass spectrometer that allows the operator to see the composition of the residual gases left in the vacuum chamber after pumping. Basic theory here.
This RGA is equipped with an electron multiplier so it operates at a maximum pressure of 1e-4Torr and is good down to about 1e-15Torr which is not a limit I will be reaching anytime soon. The spectra shown above indicates a large peak at an AMU of 18 which is H2O. This means that the chamber needs to be baked out to remove water embedded in chamber walls and surfaces until the highest peaks are N2 and O2.
A spectrum analyzer and near field probe was used to determine the main operating frequency of the RGA to approximately 3.02mhz. The RGA is also equipped with an Ar calibration source. Serial logs indicate that it was manufactured in 2007 and has been run thousands of hours past recommended service/filament replacement and the measured total pressure in the chamber was over an order of magnitude too high when compared to a hot-cathode ion gauge, but after calibration everything seems to be in spec.
Thermal evaporation deposition of Al and Ge from Tungsten and Tantalum boats, respectively. Process took place at 5.5×10^-6 Torr and around 200 amps. Aluminum alloys with Tungsten at the high temperature and causes boat failure, a thicker gauge boat will be used in the future or one made of TiB2-BN or BN. Update: W 0.015″ boat thickness seems OK.
Approx. deposition rate throughout the run was 2.2A/s, with total accumulation of 500A. Much faster than my sputtering setup but yields a far worse film.
In situ plasma cleaning is via the red ICP coil seen in the 4th picture. A Quartz Crystal Microbalance (QCM) is used to measure the thickness of the deposited films and current is supplied by a rewound microwave oven transformer. The UV-VIS spectrometer is used to monitor the emission spectra of O2 plasma. O2 is flowed into the chamber via a Mass Flow Controller (MFC) until the pressure is 75-100mTorr and the substrate is plasma cleaned for 5 minutes with 100W RF prior to depositions.
Films as thin as 0.2nm (2 monolayers) can be deposited with great care, although a shutter would make it much easier. The next revision will include a shutter and redundant QCM.
As current pass through the boats, they heat up to 1000 – 1800C and subsequently heat up much of the surrounding chamber and mounting parts. This starts serious outgassing in the chamber and without prior cleaning and bake out quickly raises the pressure to non-workable pressures and the deposition rate slows.
I added a second turbo pump to raise the pumping speed (previously 110L/s and now an additional 50L/s) and to tolerate higher outgassing.
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RF or DC oxygen plasmas are composed of a number of highly energetic and reactive species that will readily clean most organics off wafers and strip photoresist.
While operating with prolonged high power plasmas residual gas deposition and chamber sputtering collects on most surfaces.
- Clean/prep wafer – Piranha, RCA 1 / 2
- Water rinse
- Remove native and RCA oxide – 1-2% HF dip
- Field oxide growth – 2 hours @ 1200 c w/ water vapor, 5000A blue film
- If wafer in storage, dehydration bake – 10 min @ 220c
- Check wafer hydrophobic if necessary
- Optional spin HMDS
- Spin 3.5mL AZ 4210 resist 30 sec @ 3500 rpm ~3.5um film
- Soft bake resist 3 min @ 105c hotplate
- Expose active area – 18 sec DLP projector
- Develop 1:3 400k KOH:H20 puddle 2 min
- Water rinse (no solvent)
- Inspect wafer, if defect strip resist and retry
- Hard bake 5 min @ 115c hotplate
- Etch active area – 1-2% HF 10 min or until surface hydrophobic
- Water rinse
- Resist strip – Plasma ashing 100 watts RF 5 min @ 125mTorr O2
- Acetone rinse
- IPA rinse
- Water rinse
Tall particles can easily short out the thin gate oxide in these devices, as shown under my SEM. This poses an issue for making such devices in a garage; the gate oxides must be grown thicker to mitigate shorted devices which leads to a higher threshold voltage for the FET.
Recent progress into 1um feature sizes and beyond with a LCoS projector and an infinity-corrected metallurgical microscope. Given a numerical aperture of 0.98 on the microscope objective and with an exposure wavelength of 365nm the simple calculated resolution is 0.227um however the actual resolution is probably around 0.5um due to diffraction limitations inherent in this projection system. The depth of focus @ NA = 0.98 is calculated to be approximately 1.8um but is likely worse.
Laser illumination of the projector is avoided due to interference patterns. A SMD 395nm LED is used in some tests and a standard high pressure mercury arc vapor lamp in others.
Exposure times calculated by integration of total UV dosage measured at different wavelengths with the radiometer. To calculate exposure time for AZ4210 resist, for example, the datasheet is consulted to see a recommended dose of around 135 mJ/cm^2 for a 3.5um film thickness. If exposed with a 5x objective on my system, the exposure time @ 410nm is (135 mJ/cm^2)/(4.05mW/cm^2) = approx. 33 seconds. This is a bit longer than I would like but given that it is a positive resist that is to be expected.
Basic mask set for a MOSFET. Fiducials should be added to the corners for subsequent layer alignment. Active area is etched into field oxide, active area is doped, gate oxide is etched and regrown as thin a possible without pinholes, contact area is etched into oxide, then aluminum or copper is sputtered or evaporated and patterned.
A small, modular, and versatile chamber was constructed for plasma and other research experiments. The main vacuum manifold can be configured with multiple feedthroughs in either standalone vacuum mode or connected to a large chamber. The front is a 6″ CF viewport that can be swapped for a gas feedthrough assembly.
The chamber is being used right now to study simulate vacuum conductance and pressure gradients across larger chamber systems such as an ion implanter with a turbo pump on one end and an ion source and MFC gas flow far away from the pump. The gas flow creates a higher pressure in the ion source chamber and, in theory, allows for low energy beam transport and acceleration into a much lower pressure substrate/target chamber (in the case of an ion implanter).
The Micro Ion Gauge ATM is an awesome gauge that is extremely sensitive and reads from atmospheric pressure down to 1e-9 torr switching seamlessly through a range of 4 different vacuum gauges. It has an analog voltage output from 7volts down to 0.5v logarithmic to the pressure. This gauge had very bad noise issues, the voltage output moved with pressure change but was not useable due to the analog output swinging ~ +/- 0.25 volts multiple times per second.
The problem was found to be bad electrolytic caps, as expected. They exhibited high ESR, Low DC resistance, puffed up vent/tops, and electrolyte leakage on the PCB.
A quick attempt making a titanium sublimation pump. Ti welding rod was bent into a coil around aluminum round stock and placed across 30-50 amps in high vacuum yielding successful results. Chamber was roughed down to 20mTorr then pumped with turbo to 1e-5 and briefly baked out. More testing will be done and results will be posted along with an updated design with shielding to make it into an actual pump rather than depositing all over the chamber walls.
During testing I experienced a strange pumping curve a few times as shown in the last picture where the filament while heated at 50 amps seemed to out gas twice before reaching sublimation.
(Click on image to enlarge)
The basic idea of a TSP or getter pump is that the filament is heated past 900 degrees c with a high current across it. The filament first out gases and raises the chamber pressure, but then reaches sublimation pressure where it begins to form a thin volatile coating of Ti on the chamber walls.
Titanium, in this heated state, will readily combine with gaseous specious in the chamber to form a more stable coating and the gas molecules in the chamber basically get incorporated into a thin film on the chamber walls and trapped. The filament current is cycled for highest effectiveness. A rewound MOT is used for high current supply.