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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.
(Click on image to enlarge)
Progress in developing the metalization process for the home chip lab. DC and RF sputtering is used and the process will be refined more and then I will move on to the wet process with etching metal through resist mask, etc.
Sample is scratched with a razor and surface roughness is measured with a KLA Tencor Stylus Profiler. Surface is extremely rough and best interpretation of the data leads me to believe the thickness of the sputtered film is approximately 0.492um.
Two internal radiant infrared heat lamps and external resistive heating elements are used to bakeout the vacuum system for 24 hours allowing for ion pump operation. Normally when the chamber is full of unwanted moisture and impurities ionizing gas in the chamber will lead to a sharp increase in pressure due to plasma cleaning of the chamber walls and subsequent release of gas molecules.