Fabrication: Electron beam lithography system
Electron beam lithography is one of the techniques to fabricate small devices in nano meter scale.
In this method, we need electron beam lithography systems.
1. Mechanism of electron beam lithography systems
In our research field, we usually use systems utilizing spot electron beams to exposure patterns.
The basic mechanism of the systems is almost same with that of scanning electron microscopes (SEM).
The system has an electron gun to create electron beam, high voltage electrodes to accelerate electrons, electron lenses to focus electron beam, coils or electrodes to control position of the spot, a precise stage to mount samples and a control system for the components.
In this field, systems fabricated by Elionix, JEOL, Raith, Vistec, etc. are often used.
2. Electron guns
There are mainly three kinds of electron guns.
Thermal electron guns utilizing thermal electrons from tungsten or LaB6.
Filed emission electron guns utilizing tunneling effect from tungsten under high electric fields.
Schottky electron gun utilizing thermal field emission from tungsten coated with ZrO.
Systems for large pattern often use the thermal electron guns, which can realize large currents but not small beam spots.
High grade systems for small patterns and large currents use the Schottky electron guns, which can realize large currents and small beam spots.
If you make metal electrodes narrower than 100 nm, a system with the Schottky electron gun or the field emission electron gun will be a good choice.
3. Acceleration voltage
With higher acceleration voltage, you can fabricate smaller patterns.
High acceleration voltage decreases the size of the beam spot and the scattering in the resist.
We can fabricate metal electrodes with the width of 20 nm by using 75 kV acceleration voltage.
We sometimes need patterning on the other patterns.
In this case, resolutions of the stages become important.
We should design patterns by considering the resolution.
High grades systems have stages with laser interferometers, which can realize the resolutions in several tenth of nano meters.
5. Tips in using electron beam lithography systems
5.1. Adjustment of focus and stigmators
I usually use the following steps.
Rough adjustment of focus => adjustment of stigmators => fine tuning of focus
I repeat this process until obtaining good beam spots.
If you know the mechanisms of the focus controllers and the stigmators, you can adjust more easily.
You can use small gold particles or contamination created by spot baking.
Contamination is the bank of carbon which is created by ionization of hydrocarbon in the chamber and attracted by electron beams.
If you bake too long, the shape will be collapsed with too much carbon.
So you need to find a good condition depending on systems and beam currents.
In our system, 3 seconds is appropriate value in the 60 pA condition.
5.2. Height sensors
Some high grades systems have height sensors to keep the sample height at best positions in exposure processes.
These are very useful tools to write small patters.
In the adjustment processes of focus, keep the stage height at the standard position with using the height sensor.
Otherwise, the machine will exposure all patters with unfocused condition.
If you face troubles that the height sensors do not work well, the following will be the possible reasons.
A. Tilt of the sample (If you have some resist on the backside of the wafer, it will tilt the sample. Clean up the resist.)
B. Rough surface of the resist
C. Not appropriate thickness of the resist (I feel thick resists are not good.)
D. Degraded output of the laser diode in the sensor
In many cases, A, B and C will be the reason.
If all users often experience errors in height sensor, D might be the reason.
5.3. Order of shapes in exposure
I prefer to exposure from fine shapes to large shapes.
This is to prevent the effect of the stage drift.
The stage sometimes drifts after moving to the appropriate position with the laser interferometer.
If you exposure large shapes first, the stage might be shifted at the important time to exposure the fine patterns.
(In some old systems, I heard a rumor that the first period is not good because the spot drifts by discharging. This might be caused by bad handling of the blanked beam. But in new systems, I don't think so.)
The order of exposure is determined by the order of commands to draw shapes in pattern files.
You can change the order by changing the order of commands.
If you prepare a program to resort the commands, the process becomes much easier.
5.4. Dose timer
One of the important parameters in exposure is the value of the dose timer.
With considering the resolution of the hardware timer, decide the value.
For example, setting the value by 0.01 us in a machine with 0.05 us resolution is nonsense.
Also it is safe not to use too small values of dose timer to prevent errors.
I prefer the values larger than 10*resolution.
You can realize this by choosing appropriate chip sizes, dot maps and currents.
The size of the patterns is limited by the resist eventually, too large dot maps with small chip sizes are nonsense.
(In some old systems, the hardware cannot support large dot maps and automatically thin the dots.)
With understanding the system well, let's fabricate many good devices!